PURGIT mobile tank degassing does these 4 things better than any other vapor control service on the planet.

1. We can degas a tank without opening the tank up.
2. We make no pollution by burning the cargo and exhausting to the air.
3. Our work is absolutely verifiable.
4. We return the cargo as liquid for recycling.

Mobile equipment using the most sophisticated technique available for tank degassing is made possible by the PURGIT system. Clean, safe and efficient.

The problem of channeling, looking down on the tank

In Texas, the Emission Inventory (EI) declared by industrial sources does not always reconcile with the actual amount of VOC measured in the atmosphere by the TCEQ. The EI records indicate that there should be lower VOC concentrations than the TCEQ air sampling stations routinely find in the area surrounding Houston TX. Part of the reason for unexplained VOC concentration is uncontrolled emissions from tank degassing. See report from the TCEQ here. Uncontrolled emissions are a result of channeling. Tank degassing is regulated by TAC 30, 115.541.
Channeling is a condition that is best resolved by mixing. Without mixing the vapor inside the tank it is nearly impossible to degas the ‘dead spots’ in the vapor space. Yes, they will eventually weather out, but only as a result of a lot of time and the burning of a huge amount of lean vapor that will require a large quantity of supplemental fuel gas. That is a waste of time, money and resources.Thermal oxidizer systems pull air into the tank to replace the tank vapors they pull out. The replacement air comes in through vents on or around the roof and that air continues on the path of least resistance to the exit port. The plan view diagram on the left represents channeling from leg supports or vacuum breakers. This replacement air channels to the exit and that makes the vapor at the exit not representative of the vapor in the tank. Testing will show lean vapors that do not reflect the concentrated vapors in other parts of the tank.

Poor testing procedure results in premature opening of the tank, and that results in uncontrolled emissions. Mixing tank vapors should be required because that is the only way to get a sample of homogeneous, representative vapor.

Almost all hydrocarbon vapors are heavier than air.

That point is widely known, understood and accepted. These heavy hydrocarbon vapors are present in the form of ‘clouds’ that lay on the floor inside storage tanks. The clouds are intensified and regenerated by cargo on the floor. Regeneration is the term that applies to the evaporation and development of the vapor cloud inside the tank. When a combustion device is used for tank degassing, the heavy vapors below the exit port remain virtually untouched. The light replacement air takes the path of least resistance. The vapor cloud is hardly impacted when thermal devices remove tank vapors. The result is that the tank is not adequately degassed because the main body of vapors is not controlled because it is not tested. It is not tested because it is inaccessible to any system that does not mix the vapors.

Experts agree that combustion is a poor technique for tank degassing.

Side view of vapor channeling:

PURGIT mixes vapor:

The PURGIT system is the solution. PURGIT is the only degassing contractor in the country that operates with a return stream to the tank. This provides the ‘stirring’ of the vapor space that is necessary for a thorough job. Return gas = mixed tank vapor. There are no dead air places and there is no channeling because of the mixing.Mixed tank vapor gives you a representative sample of the vapor in the tank. That makes for more accurate sampling and you can know the tank vapor meets the facility and / or the government specification for open venting before you open the tank.

Consider this:

* The return vapor stream inside the tank encourages the cargo to mix and that prevents channeling.

* Mixing provides a much more uniform vapor in the tank so vapor testing is actually representative of the vapors in the vapor space.

Only condensers can effectively degas tanks while returning and mixing the tank vapor space during vapor control. That is why they are so effective.

The side benefit is that our method does not make greenhouse gases. The condenser system becomes an extension of the storage tank and no emissions are released during the vapor control. The condensate is collected and can be recycled.

How effective is the PURGIT system? It is very effective! Since we have been counting we have condensed and recovered over 140 tons of VOC for recycling.  Incidentally, 140 tons of VOC represents over 400 tons of CO2 that was not dumped into the atmosphere. We are proud of our achievement and contribution to pollution control

What does it take to do vapor control? All it takes is a sophisticated system that addresses the real vapor control problems and solves them in an environmentally correct way. The PURGIT system becomes an extension of your tank. The whole condenser system can be tested to over 10 psi. It does not have an exhaust stack. Tanks can be degassed without a permit because there are no emissions.Can we prove we are better than thermal oxidizers? Yes, because our work product is the recovered condensate available for recycling. We condensed and recovered over 140 tons of VOC in just a few months. That would have put over 400 tons of CO2 in the atmosphere if it had been burned.Burning tank vapors is not necessary for vapor control, but eliminating channeling and breaking up vapor clouds is necessary.


This is a simple flow diagram showing our system. We have the most innovative vapor control system in the industry.The PURGIT system controls emissions from your tank without releasing anything to the air and it keeps tank owners in compliance with state regulations.

We invite you to check our system out with your infra-red camera on the next job. You will not see any ugly clouds like you see coming from thermal oxidizers. The reason is that we have the only closed-vent system tank degassing system in the industry.
* What is vapor channeling? The 21st Edition of INDUSTRIAL VENTILATION published by the American Conference of Governmental and Industrial Hygienists says that vapors flowing inside a tank can and do channel. Pictured right is the 21st Edition, but the 26th Edition and many previous editions say the very same thing – that air streams do not move evenly through the tank, channeling is a real problem.Very likely you can feel the effects of vapor channeling in the room you are in right now. There is a hot or cold part of the room and then there is a place under the vent where it is too cold – or too hot. The air moves from the supply register to the return vent and it does not move to all parts of the room evenly.

And consider that your room was designed for good air flow. Now you understand the problem of channeling.

PURGIT understands how this problem applies to tank vapor control. We are the only company that has a solution to the problem of vapor channeling.

]]> http://www.purgit.com/vapor-channeling-and-tank-degassing/feed/ 0 http://www.purgit.com/ammonia-flare-service/ http://www.purgit.com/ammonia-flare-service/#comments Fri, 12 Feb 2010 01:28:23 +0000 hhilliard http://www.purgit.com/?p=650 WE flare anhydrous ammonia, LPG, hydrocarbons and other gases.

PURGIT has the equipment to safely degas ammonia tanks and pipelines. A flare with a specially designed tip is used to burn vapors. We have completed many tank decontamination and conditioning jobs on marine barges and shore storage tanks. The shore tanks can be low pressure or high pressure. We have also done work on ammonia pipelines, pig traps and small tanks in refineries.

Our mobile equipment is capable of moving to any vessel, pipeline, etc.   Here are some photos from past jobs:

ammonia flare with hot flame. Big ammonia tanks are usually degassed as part of routine maintainence so they can be opened for inspection. The procedure is to remove all liquid and then remove the ammonia gas in the tank. The tank is flooded with nitrogen gas to dilute the ammonia. The diluted ammonia and nitrogen gas mixture is vented to aflare or combustor. The vented gas is tested for ammonia concentration. The venting continues until the tank vapors are acceptable for open venting. Anhydrous ammonia is hygroscopic and introducing water into the tank too early in the cleaning phase can be detrimental to tank integrity. The addition of water to an anhydrous ammonia tank will create heat and a strong vacuum that may cause the tank to implode.	Hot flame burning ammonia We normally work for owners and contractors on big tanks and pipelines. On big tanks we connect with 4″ vapor hose. We have adapters to fit most flange sizes. We can work on pipelines and small tanks. Our standard equipment also includes 2″ and 3″ SS hose for liquid ammonia. Once the tank inspection is completed, the tanks are filled with nitrogen gas to remove air and moisture. The tank can be tested at this time for leaks. Then liquid ammonia is introduced into the tank to condition the tanks. The liquid ammonia flashes in the tank diluting the nitrogen and pushing it to the flare. The venting process continues until the non-condensible gases are low enough for the tank compressors to operate successfully.
	Tank barge ammonia job The long white tank is an ammonia tank on a tank barge. It would hold about 20,000 tons of ammonia liquid.
This is the PURGIT mobile ammonia flare in travel mode. On the back of the truck, you can see liquid propane tanks, propane vaporizer, hose, connection fittings, etc. This photo was taken as we were about to depart on a job. The ammonia equipment is usually ready to go on short notice.	Burning ammonia requires supplemental fuel. Natural gas or propane must be added to get the ammonia to burn properly. PURGIT has vaporizers like this one. And we have 420# propane tanks loaded and ready to go for supplemental fuel. The photo shows a small cylinder for pilot fuel.
Flare job with PURGIT portable flare on butane pipeline. This job was in Mont Belview near Houston TX	Another PURGIT   mobile flare burning propane in Kazakhstan.
This is another view from the job above. The PURGIT truck is in the foreground and our vaporizer trailer is behind.	A pipe flare burning propane.

General Sam Houston led the Texas Army to a victory over the Mexican Federal Army in a decisive battle on April 23, 1836. The battle took place on the banks of Buffalo Bayou, very close to the confluence of  the San Jacinto River, not far from Galveston Bay. After the Battle (now known as the Battle of San Jacinto) Santa Anna changed into the uniform of a private and ran away trying to escape the Texans. He was captured a few miles away by troops from the Republic of  Texas Army.

The capture site was on what is now the Houston Ship Channel. Back in April, 1836 it was the confluence of Buffalo Bayou and Vince’s Bayou. Santa Anna didn’t know that Def Smith and other soldiers burned the bridge at Vince’s Bayou to prevent him from escaping back to his main army.

The bridge monument (photo above) is about 3/4 mile away from the capture site monument.

The Texans went to war with Mexico because Santa Anna voided the Mexican constitution of 1824 that allowed the Texas settlers to have some independence. Santa Anna led his army into Texas  attacking the settlers. The Texans organized and formed a government and army. There were several skirmishes and battles that led up to a famous battle. Texas army occupied a mission in San Antonio called the Alamo to slow down the Mexicans. Santa Anna laid siege to the Alamo killing  General Travis, Davey Crocket and all the soldiers and volunteers. All of the Texans at the Alamo knew that Santa Anna had ordered ‘No quarter’ for prisoners meaning they would all be killed, even if captured. Following orders from Santa Anna, the Mexican General Urrea captured, disarmed and executed the Texan General Fannin and every one of his 300 of his men at Goliad. They were were massacred on Palm Sunday, March 27, 1836.

The point of the battle at the Alamo in San Antonio was to give General Sam Houston time to develop a battle plan. Santa Anna swept through Texas looking for the Sam Houston and the rest of the Texas army. He thought he had the them cornered at a site south west of Houston TX at what is now near Deer Park TX. However, the Texans made a surprise attack and routed the Mexican army. The battle cry from the Texans at the Battle of San Jacinto was ‘Remember the Alamo’ and ‘Remember Goliad’. No Texas were killed although General Sam Houston was mortally wounded. Many of the Mexican soldiers were killed or captured.

Although it was a relatively small battle in terms of combatants, the battle resulted in Texas becoming its’ own country. Texas was an independent country for about 9 years.

The battle takes its place as one of the most significant battles in history because of the land and political changes that resulted. The battle site is marked by the San Jacinto Battleground monument, perhaps the tallest and most beautiful monument in the USA. Every Texan should carry a photo evidencing them standing in front of the San Jacinto Monument at all times. I can’t find my photo, and so I am headed back down there to get another. I will post it up here soon.

After he was captured and signed a surrender agreement, Santa Anna was allowed to go back to Mexico but he continued to cause trouble. The President of the United States of America, James Polk  sent an army led by General Winfield Scott to Mexico. The US Army marched to Mexico City capturing the Mexican Federal government. The Mexican government surrendered to the United States of America after a heated, but relatively short battle. Next time you hear someone making some outrageous claim that Mexico owns California or Texas you can remind them of that.

#1 photo of the Santa Anna capture site

The actual site of the Santa Anna capture was marked in March of 1916 with the monument pictured left. This is an actual photo of the new marker shortly after it was placed. Linda Conard sent the photo and reports this:

“The man at the head of the bull is my great grandfather, Joseph Morgan Cruse and the woman on the side of the bull with hand on it is my great grandmother Zella Gregg Cruse. I have no idea who the other people are. The boy is too young to be my grandfather, Aubrey, Sr. and my Dad, Aubrey, Jr. wasn’t even born yet. The man behind the bull could be a farm worker.

The farm was owned by J.S. Cullinan and Joe and Zella managed it. Strawberries were grown there. Joe and Zella also owned a store in town.

The marker was set in March of 1916 and this photo was taken in May 1916. I believe the house remained there for many years after Crown was built. Joe and Zella are buried at Crown Hill Cemetery nearby.”

Thank you to Linda Conard for sending the photo. It is used here with her permission. Crown is an oil refinery (now called Pasadena Refining)  and it is in the background of the photo, below. Crown Hill is a salt dome and the cemetery is on top like many cemeteries. The Houston Ship Channel was not in place at the time of this photo. Buffalo Bayou would have been a small stream and the monument is facing it.

Below is a view of the Houston Ship Channel about 1999, with another view of the monument at the old Santa Anna Capture site.

The Santa Anna site reminds me of a place in New Orleans – at the Harvey Canal to be precise – on the River Road West Bank where there is a lookout over the Mississippi River. It is a great view of the Mississippi River and the city of New Orleans in the background. I better get a photo of that site before it is erased from the landscape.

Call Townsend Hilliard for more information 713-201-7517

Pictons of Houston

Recipe Pages

Divider Pages

Cookbook cover

Rockport TX

Picton Tow Boat Company

The founder of the Picton Tow Boat Company was DM Picton. He died in 1937. His obituary says in part:
“The late Mr. Picton was a pioneer on the Texas coast in the construction of rock breakwaters and jetties and other type of large and difficult marine construction. In the last month of his illness it gave him considerable satisfaction that the company, which bore his name and which he headed as president, had completed the very difficult job of capping with concrete the long east jetty at Sabine pass.
This piece of work was reputed to be the most difficult piece of concrete pouring ever accomplished, and apparently insurmountable obstacles were overcome in the capping of the 6,900 foot jetty.
He lived in Rockport for 50 years. He was the contractor who built the Corpus Christi breakwater approximately 17 years ago, and he also built the jetties at Aransas Pass, Freeport, Galveston, Sabine Pass and the southwest pass in the Mississippi River.”

The Aransas Pass jetty would allow commercial development to expand toward Rockport TX or Corpus Christi TX. Both cities wanted development to go their way. Here are some old advertisements advocating bringing business in the direction of Rockport. As it turned out, the Corpus Christi development was more successful as growth went that way. Rockport today is a small town and artist colony. It is a great place to visit and live. There are many restaurants and it is a hub for sport fishing. Catches include red drum, speckled trout, flounder, etc.

Deep Water Harbor built in Port Aransas 6.1 mb .pdf file This is an advertisement for Rockport TX. It was hoped that the jetties built at Port Aransas would make Rockport more popular. This is a very interesting advertisement.

Rockport advertisement 5.7 mb .pdf file  Another advertisement for the city of Rockport TX and the Deep Water Harbor

DM Picton and his 5 sons ran the Picton Tow Boat Company and the company continued after his death. Here are some 1950 rate quotes for the Picton towing out of Port Arthur TX
Picton rate schedule 1950 this is a .pdf file If you want to see the complete map , you will have to print and then cut and paste. It shows steaming time in the Sabine district for the Picton Tow Boat Company.

Ports that Picton towing served in 1950 .pdf file This shows the ports served by Picton Towing out of Port Arthur TX

To the best of my knowledge, the  Picton Towing vessels were sold to Moran Towing about 1977.

Propeller Terms and Definitions

1. Diameter –The diameter of the imaginary circle scribed by the blade tips as the propeller rotates.

2. Radius –The distance from the axis of rotation to the blade tip. The radius multiplied by two is equal to the diameter.

3. Blade Face –Pressure Side, Pitch Side. Aft side of the blade surface facing the stern.

4. Blade Back –Suction Side. Forward side of the blade surface facing the bow.

5. Leading Edge –The edge of the propeller blade adjacent to the forward end of the hub. When viewing the propeller from astern, this edge is furthest away. The leading edge leads into the flow when providing forward thrust

6. Trailing Edge –The edge of the propeller adjacent to the aft end of the hub. When viewing the propeller from astern, this edge is closest The trailing edge retreats from the flow when providing forward thrust.

7. Blade Number –Equal to the number of blades on the propeller.

8. Blade Tip –Maximum reach of the blade from the center of the hub. Separates the leading and trailing edges.

9. Hub –Solid cylinder located at the center of the propeller. Bored to accommodate the engine shaft. Hub shapes include cylindrical, conical, radius & barreled.

10. Blade Root –Fillet area. The region of transition from the blade surfaces and edges to the hub periphery. The area where the blade attaches to the hub.

11. Rotation (Right hand shown here) –When viewed from the stern (facing forward): Right-hand propellers rotate clock wise to provide forward thrust. Left-hand propellers rotate counter-clockwise to provide forward thrust.

12. Pitch –The linear distance that a propeller would move in one revolution with no slippage.

13. Cylindrical Section –A cross section of a blade cut by a circular cylinder whose centerline is the propeller

14. Pitch –Reference line Reference line used to establish the geometric pitch ang1e for the section. This line may pass through the leading and trailing edges of the section and may be equivalent to the chord line.

15. * Geometric Pitch Angle –The angle between the pitch reference line and a line perpendicular to the propeller axis of rotation.

16. * Controllable Pitch Propeller –The propeller blades mount separately on the hub, each on an axis of rotation, allowing a change of itch in the blades and thus the propeller.

17* Fixed Pitch Propeller –The propeller blades are permanently mounted and do not allow a change in the propeller pitch.

18.* Constant Pitch Propeller –The propeller blades have the same value of pitch from root to tip and from leading edge to trailing edge.

19;* Variable Pitch Propeller –The propeller blades have sections designed with varying values of local face pitch on the pitch side or blade face.

20.* Rake –The fore or aft slant of a blade with respect to a line perpendicular to the propeller axis of rotation.

20a. Aft Rake –Positive rake. Blade slant towards aft end of hub.

20b. Forward Rake – Negative rake. Blade slant towards forward end of hub.

21. Track –The absolute difference of the actual individual blade rake distributions to the other blade rake distributions. Always a positive value and represents the spread between individual blade rake distributions.

22.* Skew –The transverse sweeping of a blade such that viewing the blades from fore or aft shows an asymmetrical shape.

22a. Aft Skew –Positive Skew. Blade sweep in direction opposite of rotation.

22b Froward Skew –Negative Skew. Blade sweep in same direction as rotation.

23 . Cup –Small radius of curvature located on the trailing edge of blade.

24. DAR. –Developed Area Ratio is blade area expressed as the percentage of a circle shaded by the propeller.

* denotes terms that do not have a graphic representation to aid in definition.

Any cracking in the hub of a marine propeller must be regarded as serious, and any steps which can be taken to prevent such cracking are desirable. The purpose of this paper is to enumerate some of the more obvious causes of hub cracking and to suggest certain precautionary practices which can be adopted by all those who are vitally interested in the alleviation of the problem.

It is suggested that there are at least four principal causes of hub cracking. These are listed in what is perhaps the order of their importance.

1. Excessive heating at installation or removal.
2. Improper fit of key in keyways.
3. Improper fit of shaft and hub tapers.
4. Residual casting stresses.

Discussion of these suggested causes will be in the reverse order of their listing.

Residual Casting Stresses

The stresses in a casting due to shrinkage in cooling often increase, to a considerable extent, the stresses due to the operational load. The propeller manufacturer meets this problem by giving unusual care to the uniform cooling of the casting in the foundry, Commenting on this aspect of propeller manufacture Mr. A. J. Smith, Metallurgist and Assistant Foundry Superintendent, Bethlehem Steel Company, Shipbuilding Division, has said: (MODERN CASTING, Jan” 1959)

“Most propeller designs are conducive to good directional solidification as the hub section is tapered and cast with the large end up. The massive surface area of the blades … tends to dissipate heat rapidly and reduce time required for complete solidification of the casting.”

In addition to the fact that the propeller design lends itself to uniform cooling, and hence good directional solidification, the judicious use of external chills further controls the cooling. There is admittedly some possibility of hub cracking from residual casting stresses, but, because the propeller manufacturer is first and foremost a skilled foundryman, the possibility is remote and cracking from this cause must be considered the least important of the causes named.

Improper Fit of Shaft and Hub Tapers

When the fit of the tapers of a hub and shaft is once established there is little reason for this fit to be disturbed unless actual physical damage to either the hub or the shaft occurs. The fit of the hub and shaft, however, should never be taken for granted, for the possibility of physical damage to one or the other is all too great. It is to be recommended that a responsible representative of the owners or the shipyard make some positive check at each propeller change in order to assure the continued integrity of the fit.

This is particularly true if any damage has been sustained by the propeller shaft in or near the taper, or if for any reason the propeller has become loosened on the shaft; and it is equally true if, for reasons to be suggested below, the propeller hub bore has distorted. Under any of these conditions a thorough inspection of the propeller and shaft tapers should be made and remedial action should be taken before the vessel is released for return for service.

It should be pointed out that verifying the fit of the propeller and shaft is not a proper function of the propeller repair shop, but must be done at a time and place when the propeller can be actually fitted to the shaft on which it is used. Many, perhaps most, propeller repair shops do not have a full range of taper gauges, and there is little that these shops can do except to ascertain that the propeller bore is concentric at each end. Even so, any propeller repair shop can and should make a good visual inspection of the hub bore for any evidence of looseness, wear or uneven contact with the shaft. Such a visual inspection ought properly to be a part of the repair procedure, and any evidence of wear or uneven contact should be reported to the owners.

Improper Fit Of Key In Keyways

The improper fit of the key in the keyways is always a real cause of hub cracking, and for obvious reasons. If the propeller is forced over a key of improper size or one which does not fit squarely into each keyway forces of a large order will be applied to the hub bore. Even if the improper fit does not then produce hub cracking, the condition may cause a loosening of the propeller on the shaft with subsequent risk of damage to both tapers as well as excessive vibration in operation. As a result of careful investigation of numerous cracked hubs, it is the opinion of this writer that when a crack originates in the keyway it is almost certainly the result of improper key fit.

While not a part of this immediate problem, it should be noted that the use of filleted keyways can reduce the possibility of the kind of stress concentrations which have long been associated with the use of keyways with square-cut corners. This excellent practice deserves more wide-spread use.

Excessive Heating At Installation Or Removal

The most general cause of hub cracking is the application of excessive amounts of heat at the time of installation and removal of the propeller from the shaft. This stems almost entirely from the fact that the mechanics actually doing the job do not realize and have never been told how little heat is actually required to expand the propeller hub sufficiently for their purposes.

It seems likely that the actual cracking may occur in any one of three different ways. First, if excessive heat is locally applied it may cause uneven expansion and hence breakage. Second, excessive heat at installation may cause sufficient hub expansion to allow the hub to move too far forward on the taper with the result that the hub will cool at a point where shaft diameters are larger than those of the hub bore. Breakage, then, may well result. Third, the excessively hot hub may be quenched to hasten further work. This may readily cause cracking.

Cracking from excessive local application of heat is readily recognizable from the marked discoloration of the metal where the heat has been concentrated. In some extreme cases small areas will actually begin to melt. Cracking due to cooling and contraction of the hub after being allowed to move too far on the taper is much more difficult to recognize, and the location and nature of the crack may cause it to be confused with a crack caused by poor taper fits or even improper key fit.

Heating of the hub for removal, particularly of larger propellers, may be regarded as having some justification. The only real problem is that the heat be regulated and that persons entrusted with this task be instructed as to the amount of heat required to do the job. In this case the least amount of heat is usually the best amount. Heating the hub for installation, on the other hand, must be regarded as wholly unnecessary. There is nothing to justify this practice. A properly machined shaft and hub, when clean and free of any burrs, can be tightened to any acceptable standard by use of the shaft nut alone. When the hub is heated and shrunk on the shaft tremendous hoop stress results so that the hub is operating under a stressed condition that is wholly unwarranted and for which it was never designed. No propeller manufacturer can or will assume any liability under such circumstances and the vessel owner should realize that he indulges in the practice at his own risk.

Heating in shipyard is usually with the oxy-acetylene torch. The oxy-acetylene flame has a  temperature of very nearly 6000oF. Propeller metals melt at temperatures far below this. For example, manganese bronze melts at about 1800oF., while a typical austenitic stainless steel melts at slightly more than 2600oF. The amount of heat available in the oxy-acetylene flame is, therefore, far in excess of the needs and the greatest of care should therefore be exercised in its application. The use of a standard welding or cutting torch for heating should be avoided. Because of the nature of the welding or cutting tip the flame is tightly concentrated, and the result may be a localized application of an amount of heat far beyond the requirements for loosening the propeller on the shaft.

That extremely high temperatures are not required for loosening a propeller on the shaft is made evident by an examination of the expansion rates for various alloys. It should first be borne in mind that alloys expand relatively more in the temperature range up to 212oF. than they do at higher temperatures. The chart below indicates the relative expansion of a medium carbon low alloy steel, an austenitic stainless steel, a manganese bronze and a nickel-aluminum bronze.

Since the tolerance for the small end diameter of propeller hubs, as prescribed by the SAE standards, is only plus or minus one-thousandth of an inch, it can be seen that the hub need not be expanded more than a few thousandths of an inch to cause it to become loosened from the shaft. Because this paper has been intended to be nontechnical and non-mathematical no calculations of any kind are being submitted, but anyone desiring to do so may calculate the mechanical effects of heating by making use of the appropriate tables appearing in any of a great number of handbooks.

The above chart indicates that some alloys expand far more than others at the same temperature. This does not mean that the steel, for example, must be brought to a still higher temperature, for it can be readily shown that sufficient expansion of a propeller hub can be obtained within this temperature range. What it does mean is that mangenese bronze may require less heat to accomplish the desired expansion.

Recommendations and Conclusions

We know that whatever the desired temperature may be it cannot be produced uniformly over the entire surface of the hub merely by the application of heat from a torch. However, it is necessary to apply the heat as uniformly as circumstances permit, not allowing the torch to dwell over-long in any one place but being certain that it is passed from end to end of the hub and from one portion of the hub to another in succession. The surface temperature should not exceed about 200oF. for any propeller alloy.

Additional Considerations

No mention has been made of the possibility of hub cracking from impact. The nature of the propeller blade is such that it tends to absorb the greater part of the impact sustained by the propeller, and there is nothing in the literature of propeller design to indicate that the propeller hub is ever considered vulnerable to damage from impact alone.

There seems to be little value in increasing the hub diameter beyond some optimum which is generally considered to be about 15%. of the propeller diameter. Most propeller manufacturers do not recommend that the shaft diameter exceed 60%. of the hub diameter, although one nationally known manufacturer has used smaller hubs for years with excellent results. Not only are exceptionally large propeller hubs detrimental to performance in most cases, but it is a fact that the mechanical properties of most castings decrease as the section size increases. It must be concluded, therefore, that hub size has relatively little to do with hub cracking.

While some hub cracking can be sufficiently repaired as to give acceptable service life, the practice of hub repairing by any method now known to this writer is not to be recommended. The factors which argue against the practice are: residual welding stresses which cannot be properly relieved; distortion of the bore either from its opening up as a result of the crack, or because of warpage related to the welding. process; and, finally, the unfavorable ratio of repair cost to guaranteed service life. Hub repairs can be and are being made, and propellers with repaired hubs have been known to last indefinitely, but the fact still remains that such successful repairs are more often the result of a happy conjunction of events than the predictable skill of the operator making the repairs.

Conclusions and recommendations

From the foregoing discussion it can be seen that the principal causes of propeller hub cracking are excessive heating at installation and removal of the propeller, improper fit of the key in the keyways, improper fit of shaft and hub tapers and residual casting stresses. It has been shown that of these causes the most general and the most serious is that of excessive heating. It must be concluded that cracking for this reason continues only because mechanics doing the work do not know and have not been told how little heat is required. From these conclusions certain recommendations can be made which would divide the responsibility among the propeller manufacturer, the propeller repair shop, the shipyard and the propeller owner.

1. A propeller should never be heated for the purpose of installation. The propeller hub is not designed to be shrunk on the shaft and such practice is wholly unnecessary if the tapers are clean and free of burrs or irregularities.

2. The propeller key should never be allowed to bottom in the propeller keyway. It is a good rule to allow from .020″ to .030″ clearance between the key and the bottom of the propeller keyway. Keys should be made accordingly and owners should arrange for proper inspection of propeller keys and appropriate re-work when necessary.

3. The use of a propeller puller should become standard practice in every shipyard. Every propeller having a hub small end diameter of sufficient size to accommodate such bolts should be drilled and tapped for two puller bolt holes. Owners of propellers now in service should have those propellers drilled and tapped. Propeller manufacturers would be well advised to include such holes in new propellers, and to adopt standards for or at least recognize the relationship of bolt hole and propeller hub or shaft diameters.

4. Shipyards should thoroughly acquaint themselves with the implications of this problem and take such precautionary steps as may be necessary in the individual yard. The first step is to investigate current propeller removal practices in the light of this new knowledge of the problem and to provide such instruction as may be necessary to inform employees and to remedy any present unfavorable practices. The second step is to be certain that proper heating equipment – which would be available in almost every shipyard – is on the job site and in use for propeller removal. A third step is to encourage owners to have their propellers drilled for puller bolts and to show them that this small one-time cost will have long-range benefits. Finally, the shipyards should realize that the money spent for a strong hydraulic or mechanical universal puller will be the means of saving costly man-hours, and that the time and material involved in applying excessive amounts of heat can be saved.

5. Propeller repair shops should incorporate in their repair procedures a systematic visual inspection of the hub and bore of every propeller for any evidence of uneven contact or looseness on the shaft, for any evidence of bore distortion or lack of concentricity, and for evidence of physical damage to the bore or keyway which might prevent proper fit on the shaft at the next installation. Such shops should also inform owners of the benefits of using the bolt-type puller and encourage them to have their propellers drilled and tapped accordingly.

6. Owners should come to the realization that the fit of the propeller and shaft cannot be taken for granted. Regardless of the reliance of the owner on the judgment of either the shipyard or propeller repair shop, the fact remains that the ultimate responsibility rests with the owner or his representative to require that the necessary inspections be made and to require of the shipyard or propeller shop a report of their findings. When propeller shaft damage has been sustained the owner should require a precautionary re-fit of the shaft and propeller for his own protection – and he should be prepared to pay for the cost of such additional service. If the owner hauls his vessel at various yards and is not sure that proper puller equipment is available he should provide his own and carry it aboard.

It was the announced purpose of this paper to enumerate some of the more obvious causes of propeller hub cracking and to suggest certain precautionary practices which can be adopted by all concerned. If the paper has at all accomplished these ends then it can be counted as being of some slight value to the industry.

This paper has been prepared by L. L. WALKER, JR.
Houston, Texas,  as a service to the marine industry LL Walker, Jr. had one of the first, if not the first, propeller shop in Houston, TX. In early days he worked on airplane propellers. He was well respected in the marine business. He desired to contribute to the professionalism of the shipyards by offering advice on good installation practice. I found this paper in some old files, and it is as true today as it was when it was written. I do not know when that was, but my guess was about 1970.

by Sylvan H. Wittwer

From the Fall 1992 issue of Policy Review

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One of the best-kept secrets in the global warming debate is that the plant life of Planet Earth would benefit greatly from a higher level of carbon dioxide (CO2) in the atmosphere.

You read that correctly. Flowers, trees, and food crops love carbon dioxide, and the more they get of it, the more they love it. Carbon dioxide is the basic raw material that plants use in photosynthesis to convert solar energy into food, fiber, and other forms of biomass. Voluminous scientific evidence shows that if CO2 were to rise above its current ambient level of 360 parts per million, most plants would grow faster and larger because of more efficient photosynthesis and a reduction in water loss. There would also be many other benefits for plants, among them greater resistance to temperature extremes and other forms of stress, better growth at low light intensities, improved root/top ratios, less injury from air pollutants, and more nutrients in the soil as a result of more extensive nitrogen fixation.

This good news about carbon dioxide has been all but ignored in alarmist discussions about possible global climate changes. CO2-related benefits were barely mentioned at the Earth Summit in Rio de Janeiro in June, where the rising level of carbon dioxide and other “greenhouse gases” was decried as the world’s greatest environmental threat. The Rio Summit ended with the United States and over 150 other nations signing a Framework Convention on Climate Change, committing themselves to stabilizing emissions of CO2 and other greenhouse gases at 1990 levels.

Indeed, the conventional wisdom in public policy circles is that carbon dioxide is a terrible pollutant that threatens the fate of the earth. Senator Albert Gore, the Democratic vice-presidential candidate, calls for stiff “carbon taxes” on the burning of fossil fuels, and has written in his book Earth in the Balance that the process of “filling the atmosphere with carbon dioxide and other pollutants…is a willful expansion of our dysfunctional civilization into vulnerable parts of the natural world.”

On the Republican side, William K. Reilly, President Bush’s administrator of the Environmental Protection Agency, defends the $20 billion in costs that the Clean Air Act of 1990 has imposed on the U.S. economy, in part on the grounds that it “will eliminate 56 billion pounds of pollutants annually, many of them greenhouse gases.”

And yet, for over 100 years, nurserymen have been adding carbon dioxide to their greenhouses to raise the yields of vegetables, flowers, and ornamental plants. And for decades, it has been well known among botanists, biochemists, agriculturalists, and foresters that a shortage of carbon dioxide is the most common limiting factor preventing photosynthesis from proceeding more efficiently.

The Global Warming Debate

There is no question that the carbon dioxide level in the atmosphere has been rising, and that this rise is due primarily to the burning of fossil fuels and to deforestation. Measured in terms of volume, there were about 280 parts of CO2 in every million parts of air at the beginning of the Industrial Revolution, and there are 360 parts per million (ppm) today, a 30 percent rise. The annual increase is 2 ppm, and rising. If present trends continue, the concentration of CO2 in the atmosphere will double to about 700 ppm in the latter half of the 21st century. This increase would not be a direct threat to human life; the threshold in mine-safety regulations is 5,000 ppm of carbon dioxide. But a man-made change of this magnitude in the atmosphere requires careful efforts to understand its consequences.

A number of climatologists have used computer models to predict a rise in global temperatures of 2 to 9 degrees Fahrenheit over the next century as a result of this projected rise in CO2 and other greenhouse gases, such as methane. The global warming hypothesis is disputed by many climate scientists. Even so, it is understandable that so many environmentalists and citizens would be concerned by a doubling of atmospheric carbon dioxide levels. In so changing the composition of the air, humans are inadvertently conducting a global and environmental experiment without a clear knowledge of the outcome.

While scientists disagree about the likely effects of additional carbon dioxide on global temperature, they generally agree on another important effect of a rise in the CO2 level. A doubling of the carbon dioxide concentration in the atmosphere, as is projected, would increase plant productivity by almost one-third. Most plants would grow faster and bigger, with increases in leaf size and thickness, stem height, branching, and seed production. The number and size of fruits and flowers would also rise. Root/top ratios would increase, giving many plants better root systems for access to water and nutrients.

More Efficient Photosynthesis

There are two important reasons for this productivity boost at higher CO2 levels. One is superior efficiency of photosynthesis. The other is a sharp reduction in water loss per unit of leaf area.

Photosynthesis converts the renewable energy of sunlight into energy that living creatures can use. In the presence of chlorophyll, plants use sunlight to convert carbon dioxide and water into carbohydrates that, directly or indirectly, supply almost all animal and human needs for food; oxygen and some water are released as by-products of this process. The principal factors affecting the rate of photosynthesis are a favorable temperature, the level of light intensity, and the availability of carbon dioxide. Most green plants respond quite favorably to concentrations of CO2 well above current atmospheric levels.

A related benefit comes from the partial closing of pores in leaves that is associated with higher CO2 levels. These pores, known as stomata, admit air into the leaf for photosynthesis, but they are also a major source of transpiration or moisture loss. By partially closing these pores, higher CO2 levels greatly reduce the plants’ water loss–a significant benefit in arid climates.

There are marked variations in response to CO2 among plant species. The biggest differences are among three broad categories of plants–C3, C4, and Crassulacean Acid Metabolism or CAM–each with a different pathway for photosynthetic fixation of carbon dioxide.

Most green plants, including trees, algae, and most major food crops, use the C3 pathway, so named because the first products of photosynthesis (called photosynthate) have three carbon atoms per molecule. C3 plants respond most dramatically to higher levels of CO2 . At current atmospheric levels of CO2, up to half of the photosynthate in C3 plants is typically lost and returned to the air by a process called photo-respiration, which occurs simultaneously with photosynthesis in sunlight. Elevated levels of atmospheric CO2 virtually eliminate photo-respiration in C3 plants, making photosynthesis much more efficient. High CO2 levels also sharply reduce dark respiration (the partial destruction of the products of photosynthesis during nighttime) among C3 plants.

Corn, sugarcane, sorghum, millet, and some tropical grasses use the C4 pathway, so named because the first products of photosynthesis have four carbon atoms per molecule. C4 plants also experience a boost in photosynthetic efficiency in response to higher carbon dioxide levels, but because there is little photo-respiration in C4 plants, the improvement is smaller than in C3 plants. Instead, the largest benefit C4 plants receive from higher CO2 levels comes from reduced water loss. Loss of water through leaf pores declines by about 33 percent in C4 plants with a doubling of the CO2 concentration from its current atmospheric level. Since corn and other C4 plants are frequently grown under drought conditions of high temperatures and limited soil moisture, this superior efficiency in water use may improve yields when rainfall is even lower than normal.

The lowest response to higher CO2 levels is usually from the CAM plants, which include pineapples, agaves, and many cacti and other succulents. Like the C4 plants, CAM plants do not undergo photo-respiration. CAM plants are also already well adapted for efficient water use. Under arid conditions they fix carbon dioxide at night, when the stomata are open and water loss is minimal. During the day their stomata are closed, and stored CO2 is released so photosynthesis can proceed. However, some CAM plants follow the C3 pathway when they are not under water stress; thus, succulents that receive plenty of water experience higher productivity at elevated levels of carbon dioxide.

Thousands of Experiments

Thousands of scientific experiments have been conducted to measure the effects of carbon dioxide enrichment on specific plants. In most green plants, productivity continues to rise up to CO2 concentrations of 1,000 ppm and above. For rice, the optimal CO2 level is between 1,500 and 2,000 ppm. For unicellular algae, the optimal level is 10,000 to 50,000 ppm.

Bruce Kimball, a research leader of the Water Conservation Laboratory of the U.S. Department of Agriculture in Phoenix, Arizona, has pulled together nearly 800 scientific observations from around the world measuring the response of food and flower crops to elevated CO2 concentrations. The mean (average) response to a doubling of the CO2 concentration from its current level of 360 ppm is a 32 percent improvement in plant productivity, with varied manifestations in different species.

Greenhouse-grown vegetables, including tomatoes, cucumbers, and lettuce, show earlier maturity, larger fruit size, greater numbers of fruit, a reduction in cropping time, and yield increases ranging from 10 to 70 percent, averaging 20 to 50 percent.

Greenhouse-grown flower crops, including roses, carnations, and chrysanthemums, grow to earlier maturity, and have longer stems and larger, longer-lived, more colorful flowers. Yield increases range from 9 to 15 percent, with a mean of 12 percent.

Flowers and ornamental plants propagated by cuttings, such as geraniums and a number of herbaceous and woody species, show faster and more extensive rooting, with greater plant heights and dry weight. There are also significant reductions in the time needed to grow a marketable product.

Cereal grains with C3 metabolism, including rice, wheat, barley, oats, and rye, show yield increases ranging from 25 to 64 percent, resulting from a rise in carbon fixation and reduction in photo-respiration. Flag leaves, the ones closest to grain panicles or heads, show a 60 percent increase in photosynthetic rates.

Food crops with C4 metabolism, including corn, sorghum, millet, and sugarcane, show yield increases ranging from 10 to 55 percent, resulting primarily from superior efficiency in water use.

Tuber and root crops, including potatoes and sweet potatoes, show dramatic increase in tuberization (potatoes) and growth of roots (sweet potatoes). Yield increases range from 18 to 75 percent.

Legumes, including peas, beans, and soybeans, show yield increases of 28 to 46 percent. For soybeans, frequently planted not only for their food value but because they naturally fertilize the soil, there is a spectacular increase in biological nitrogen fixation, as will be shown below.

Aquatic plants are commonly limited in their growth by a shortage of carbon dioxide. CO2 enrichments induce three- to five-fold increases in algal biomass when light and mineral nutrients, especially phosphorus, are plentiful. In lakes and ponds with severe pollution, a higher CO2 concentration may therefore accentuate the danger of oxygen depletion through a buildup in algae population. This could lead to the death of fish, as happened in Lake Erie until there was a major reduction in pollutants, including phosphates. Aquatic species with foliage above the water, such as the water hyacinth, produce about 40 percent more biomass when the CO2 concentration is doubled. Such an increase could aggravate an already serious weed problem in fresh water lakes and streams.

Flower Power

The benefits of carbon dioxide enrichment are well-known among commercial growers of flowers and vegetables. Paul Nielsen, a rose grower in Santa Barbara, California, puts carbon dioxide into his greenhouse every morning before the vents are opened. “We find that roses respond well to carbon dioxide levels of 1,000 to 1,100 ppm,” says Nielsen. “The stems are larger and thicker, and there are more stems per plant.”

Dick Oglevee, of Connellsville, Pennsylvania, is America’s largest grower of geraniums. From the beginning of September through the middle of May, he burns natural gas in his greenhouses to keep the CO2 concentration at 1,000 ppm. “More carbon dioxide gives us stronger stalks, better breaking, and more cuttings per week,” says Oglevee. “I wish we could do this in the summer too, but it’s too hot to keep the vents closed.”

Richard Gerhart, a cucumber and tomato grower in North Ridgeville, Ohio, is also a big fan of CO2 enrichment: “Half the dry matter in a tomato or cucumber is carbon, and the only place that comes from is carbon dioxide in the air.” Gerhart maintains a 600 ppm concentration in his greenhouses, except on cloudy days, when he gets even better results by raising the concentration to 800-1,000 ppm.

American commercial greenhouses have used carbon dioxide fertilization for tomatoes, lettuce, cucumbers, flower and foliage plants, and bedding plants for at least 30 years. The benefits of this enrichment were first discovered by nurserymen in Germany 100 years ago, and the practice is widely used in Sweden, Denmark, Holland, Germany, Australia, and Japan, as well as the United States and Canada. Carbon dioxide enrichment is economical when greenhouse vents can be closed. It is therefore used most often in winter in northern areas and in the southerly latitudes of the Southern Hemisphere.

It is also standard practice for laboratory scientists working with algae cultures to conduct their research in CO2-enriched environments. “Most experiments with algae are conducted at CO2 concentrations of up to 20,000 ppm,” says N. E. Tolbert, professor emeritus of biochemistry at Michigan State University. “Cultures that can be grown in three days at high levels of CO2 would require 10 to 14 days at the normal atmospheric concentration.”

Phenomenal Response in Trees

Some of the most convincing evidence that the rising level of atmospheric carbon dioxide is good for plants comes from the response measurement of individual trees and overall forest growth. Forests cover approximately one-third of the earth’s land area, and account for two-thirds of global photosynthesis. They have C3 metabolism, and, like other C3 plants, respond favorably to higher concentrations of carbon dioxide.

Trees and their seedlings grown under controlled environments or in open top chambers simulating the outdoors have shown remarkable growth responses to elevated levels of CO2. Practically every species evaluated thus far in the seedling stage has shown a positive response. Addition of carbon dioxide to black walnut seedlings–at concentrations of 1,000 to 2,000 ppm for three months–increases dry weight by 80 percent, height by 96 percent, and leaf area by 79 percent. Similar results have been obtained for sugar maple, oak, ash, sweet gum, pine, and eucalyptus. The forestry department at Michigan State University has produced plantable trees in months, rather than years, by subjecting seedlings to 1,000 ppm CO2 concentrations under optimal conditions of light, temperature, day length, and nutrients.

The Water Conservation Laboratory of the U.S. Department of Agriculture has compared the growth of orange trees under the current atmospheric CO2 concentration of 360 ppm, and a concentration of 650 ppm. The trees at the elevated levels have accumulated 2.8 times more biomass in five years, and in their first two years of production produced 10 times more oranges.

Breath of the Biosphere

If plants respond so well to additional carbon dioxide, then we would expect to see positive responses to the substantial increase in atmospheric CO2 over recent decades. Several pieces of evidence suggest exactly such a response.

A fascinating report was published by scientists with the Finnish Forest Research Institute in the April 3, 1992 issue of Science magazine. The researchers reported a 25 to 30 percent increase in the growing stock of forests in Austria, Finland, France, Sweden, Switzerland, and West Germany between 1971 and 1990, and attributed this growth in part to a 9 percent increase in atmospheric carbon dioxide during the same period.

Hartwell Allen, a plant scientist with the U.S. Department of Agriculture’s Agricultural Research Service in Gainesville, Florida, has estimated that soybean yields have increased by 13 percent because of a rise in global carbon dioxide concentration. Another source of dramatic evidence of rising biological productivity comes from the increasing amplitude of the atmosphere’s CO2 cycle. The majority of the terrestrial vegetation of the earth is in the Northern Hemisphere, so more photosynthesis takes place in the spring and summer than in the fall and winter. The carbon dioxide level in the atmosphere begins to fall in the spring, and continues to fall through the summer months, as CO2 is removed by the vegetative cover of the north. In late autumn, at least in northern latitudes, much of this vegetation dies and decomposes, and photosynthetic activity drops to a low level; much of the carbon sequestered over the growing season is then returned to the air as CO2 .

This oscillation in the CO2 level is called the “biosphere’s breath,” and its amplitude is measured by the Mauna Loa recording station in Hawaii. When the first records were made in 1959, there was a 6.5 ppm difference in CO2 concentration between summer and winter. Last year, the difference was 7.5 ppm. What this means is astonishing: during their major growing seasons, the plants of the Northern Hemisphere have been able to sequester at least 15 percent more carbon than they did 33 years ago.

Other Important Benefits

Besides greater efficiency in photosynthesis and a reduction in water loss, higher levels of carbon dioxide provide other important benefits for plants.

Rising levels of CO2 compensate for the deficiencies in light that frequently occur in the winter months in northern Europe, Canada, and the United States. Indeed, flowers and vegetables grown in CO2 -enriched greenhouses experience an even higher-percentage boost in plant productivity under very low light intensities than under normal light.

Enrichment of the air by carbon dioxide also appears to offer some protection to plants against both extremely hot and cold temperatures. There is also evidence that high atmospheric levels of CO2 raise the optimal temperature for plant growth. The implication of this for the global warming debate is significant: if the higher-CO2 world of the future leads to higher temperatures, plants will respond favorably both to increases in carbon dioxide and to the warmer conditions.

Plant responses to a higher carbon dioxide concentration do appear to be limited by deficiencies in nitrogen and other mineral nutrients. If plants are to take full advantage of future CO2 -enriched atmospheres, it may be necessary to apply more fertilizer in many parts of the world. Even so, higher CO2 levels have a remarkably stimulatory effect on biological nitrogen fixation by legumes, such as soybeans. A classic study by Ralph Hardy and U. D. Havelka, published in Science in 1975, showed that a tripling of atmospheric CO2 results in a six-fold increase in biological nitrogen fixation–from 75 to 425 kilograms of nitrogen per hectare–by rhizobial bacteria in nodules attached to the roots of soybeans.

Elevated concentrations of CO2 also offer protection against air pollutants. The partial closing of the stomata at higher CO2 levels reduces the exposure of both C3 and C4 plants to ozone, sulfur dioxide, nitrous oxides, and other harmful substances in the air. The benefits are particularly pronounced for soybeans and other legumes that are especially sensitive to air pollutants.

Good News for the Planet

The benefits to plants that would result from a doubling of the carbon dioxide concentration do not necessarily mean that such a doubling is good for the planet. We do not know what the optimal level of atmospheric carbon dioxide should be. So many variables could be affected by a major increase in CO2 including temperature and a redistribution of water resources, that the honest observer has to conclude he does not really know what will happen. Even so, the good news about plant growth makes it possible to project a number of features of the global ecosystem in the next century.

First, we can expect a rapid expansion of food production that may offset some of the presumed adverse climate effects. As crop yields rise with higher CO2 levels, the amount of land devoted to agriculture can decline. It will be much easier to protect environmentally sensitive land areas from over-cultivation for crops.

Since C3 plants will benefit somewhat more than C4 plants from higher CO2 levels, there will be some shift in the mix of plants. Trees are C3 plants, so we can expect more rapid reforestation and an enormous expansion in forest biomass. Of the 21 most important food crops, 17 have C3 pathways. They include rice, wheat, barley, oats, rye, soybeans, field beans, mung beans, cowpeas, chickpeas, pigeonpeas, potatoes, sweet potatoes, cassava-yams, sugar beets, bananas, and coconuts. The exceptions are corn, sorghum, millet, and sugarcane, which have C4 pathways, and which will probably decline in relative production. On the other hand, since 14 of the 18 most noxious weeds are C4 plants, rising levels of atmospheric CO2 will generally favor crop production over weeds.

Plants, directly or indirectly, provide 95 percent of the total food of the earth. Since plants are at the bottom of the food chain, a boost in plant production should lead to major increases in bird, fish, and mammal populations as well.

The rising carbon dioxide concentration in the atmosphere must be viewed with caution. But it is inappropriate for public discussion of the issue to focus only on the hypothetical dangers of global warming that might result from higher carbon dioxide levels. It is important to stress as well the known benefits of a higher carbon dioxide concentration for the productivity of food crops, trees, and other plants.

SYLVAN H. WITTWER, professor emeritus of horticulture at Michigan State University, directed the Michigan Agricultural Experiment Station for 20 years, and chaired the Board on Agriculture of the National Research Council. He is the author of the world’s leading textbook on greenhouse vegetables, and is co-author, most recently, of Feeding a Billion: Frontiers in Chinese Agriculture.