SEALING HOT OIL SA015
The largest user of hot oil pumps is the heat transfer oil customer. Many consumers use these products with oil temperatures exceeding 500° Fahrenheit (260° C) and 600° to 700° F (315° to 370° C) becoming common. Some hotels have recently installed these systems in their laundry to dry clothing.
Heat transfer oils have many advantages over the steam that was formally used in these applications.
- The product does not flash.
- No boiler blow-down.
- No deaeration heat loss.
- No high-pressure. This means it is not only safer but also tends to leak less.
- No licensed boiler operator needed.
- The temperature can be kept uniform over a large processing area.
- You can heat and cool with the same system.
- These oils are excellent in systems that are water/ steam sensitive.
- The product is kept in a closed system. This means that all leakage can be stopped.
- There is less corrosion in the system.
In addition to these heat transfer oils you will encounter hot petroleum oil applications in refineries and hot organic oil applications in various other industries. There are several problems associated with sealing these hot oil products and each of them has to be solved if satisfactory seal life is ever to be obtained.
- High temperature oil is generally too hot for most commercially available elastomers. (the rubber parts)
- The product “cokes”.
- These coke particles form at the elevated temperatures and coat them selves inside the system piping, hardware and on the mechanical seal working parts.
- The “coke” particles restrict the movement of sliding and flexing seal components causing the lapped seal faces to open.
- The amount of coke that forms is a function of time and temperature. In other words coking will be a more severe problem in a closed loop system than it will be in the oil refining business.
- Contrary to popular opinion, testing has shown that air or oxygen is not needed for the formation of coke. This means that seal designs that try to eliminate oxygen by quenching or some other method will not work. The use of steam quenching is limited to its cooling effect only.
- The product is always a fire hazard and depending upon the type and brand you purchase there could be toxicological problems. Keep in mind that the seal is going to wear out or fail at some time and the product is going to leak out to the atmosphere.
- Thermal growth of the pump parts will cause problems in maintaining proper pump “wear ring” and impeller clearances as well as the correct seal compression.
- A good rule of thumb to remember is that each inch of shaft will grow both axially and radially about 0.001 inches / inch of shaft for each 100°F. rise in Temperature, or 0.001 m m./mm. of shaft for each 50°C rise in temperature. This can have a profound affect on impeller to volute clearance.
- Misalignment between the driver and the pump and between the piping and the pump suction is a serious problem at elevated temperatures.
- The product is costly. Leakage represents large monetary losses and personnel danger as well as environmental problems.
- Heat tracing must be provided throughout the system to prevent the product from becoming too viscous during periods of prolonged shut down. Unfortunately no one ever heat traces the stuffing box.
- Vibration is always a problem with hot oil pumps because the coke attaches to rotating components interfering with the dynamic balance.
- You always end up pumping slurry, which means frequent impeller adjustments or wear ring replacement and excessive vibration due to the imbalance caused by wear of the rotating parts.
- As the coke builds up on the inside of the discharge piping the pump will operate further off of its best efficiency point (BEP) causing shaft deflection, vibration, and excessive seal movement. Coking on the inside of the suction piping can also cause cavitation problems
Although there are many techniques available to address each of these problems, the combination of these problems eliminates most of the common techniques and leaves the customer with very few options to get good seal life. Regardless of the seal selected you must address all of the problems or the seal life will be shortened.
Oil refineries pump hot oil with closed impeller pumps and as a result have to put up with the additional problems associated with replacing “closed impeller” wear rings. Unlike the chemical industry they cannot take advantage of the features of an open impeller design that can be easily adjusted to maintain maximum efficiency. There are two reasons why oil refineries chose closed impeller designs with mechanical seals and API (American Petroleum Institute) glands:
- Fear of a bearing failure that could cause sparking as the metal impeller contacted the metal volute. The soft non sparking metal wear ring on one end of the shaft and the carbon disaster bushing installed in the API (American Petroleum Institute) gland on the other would insure no hard metal contact if a bearing failed as the shaft was turning.
- Shaft expansion or impeller adjustment could cause the rotating, open impeller to contact the stationary volute. To prevent sparking, the impeller or volute would have to be manufactured from a soft non-sparking metal such as aluminum or bronze and this would not be very practical. Hence the closed impeller with the soft wear rings
To insure long seal life you must do the following:
The product has to be cooled in the seal chamber:
- The oil must be cooled to stop the coking. Coke is a function of heat. Many years ago it was believed that oxygen had to be present for coking to occur, but testing has shown that this is not true. You can coke any petroleum product in an inert atmosphere as long as the temperature is high enough. The finest lubricating oil available will start to coke at 300° F (150° C). The oil temperature and time determine the amount of coking that you get.
- The oil must be cooled to prevent damage to any elastomers that might be installed in the seal or shaft sleeve. Elastomers that are subjected to high heat will first take a compression set and then shrink in volume. They will eventually grow hard, crack and leak excessively.
- The oil must be cooled to reduce the amount of heat that will be transferred through the shaft to the bearing oil or grease. This heat will reduce the viscosity of the lubricating oil or grease and eventually cause premature bearing failure. The SKF bearing company states in their lubrication literature, that the life of bearing oil is cut in half for each ten degrees Centigrade (18° F) increase in bearing oil temperature. They recommend 60° C to 70° C (140° F to 158° F) as an ideal oil temperature.
- The grease or lip seals are sensitive to any increase in shaft temperature. A stainless steel shaft is a good choice in these applications because stainless steel is a poor conductor of heat compared to carbon steel. This is the reason there are no stainless steel frying pans unless they are clad with either aluminum or copper.
You must install a back up seal for the following reasons:
- The product is dangerous. Leaking hot oil can start a fire or injure any personnel in the area. Many brands are toxic and some have been identified as possibly carcinogenic.
- The product is too costly to tolerate even small amounts of leakage.
- Back up cooling is necessary if the primary cooling method fails. A back up seal, with a cool barrier fluid system, can provide this cooling
- If you elect not to use a back up seal, then be sure to install an American Petroleum Institute (API.) type gland.
Take a look at the illustration below. This is an API (American Petroleum Institute) gland that can perform several functions:
- The disaster bushing (DB) can provide shaft support if you lose a bearing.
- The leakage will be directed to the quench and drain connection (Q) when the seal wears out or fails.
- The quench connection (Q) will allow you to use steam for product cooling, but do not use too much because it could penetrate into the bearing case.
- You can connect steam to the quench connection and use it to put out a fire, should it occur on the outboard side of the seal.
- In this application the flush connection (F) is not used. The stuffing box is “dead ended” to take full advantage of the heating/ cooling jacket.
Whenever possible a large diameter cooled sealing chamber should be installed on the pump:
- To allow room for centrifugal force to throw solid coke particles away from the seal faces and sliding, or flexing components
- Misalignment is always a problem in these pumps. This shaft displacement can cause the rotating seal to rub against stationary parts in a conventional stuffing box.
- Vibration means movement. The seal must be free to move within the seal chamber.
- When the pump stops gravity will pull solid particles to the bottom of the stuffing box. A large seal chamber will almost guarantee that the particles will not collect around the seal at this time.
A cartridge seal is necessary in most applications.
- Thermal growth will cause volute, casing and shaft expansion. Only a cartridge seal can compensate for this movement and allow for the impeller adjustment that will be necessary.
- The wear caused by the slurry will cause frequent impeller adjustments. The average pump used in these applications has almost 0.250 inches (6 mm) of adjustment possible.
To compensate for misalignment you will have to:
- Use a “C” or “D” fame adapter to compensate for misalignment between the pump and its driver.
- These adapters are available from all good pump companies and will compensate for misalignment as the pump goes through its temperature transients.
- No other method of alignment works anywhere near as well. If you are going to do a conventional alignment with dual indicators or a laser aligner be sure your calculations compensate for thermal growth.
- Use a “centerline” wet end to prevent excessive wear ring wear and pipe strain at the pump suction. If your pump did not come equipped with this type of wet end it can easily be installed in the maintenance shop. Look at the following illustration:
|The centerline wet end has the feet attached to the sides of the pump at the centerline instead of the bottom of the pump
The centerline design allows the hot volute to expand up and down and eliminates a lot of pipe strain due to thermal expansion.
Now that we have discussed these important points let’s take a look at some solutions that are often offered, but we should not adopt as our solution. Here are the things that do not work well:
Bad solution #1.
- Use a metal bellows seal to eliminate the need for cooling in the seal area. Although the metal bellows does not have rubber parts that are sensitive to high temperature cooling is still needed for the coking. Most bellow suppliers offer an A.P.I. type gland to provide low-pressure steam behind the seal for cooling purposes and thereby eliminate the option of backup sealing. This quenching should be limited to only a back up cooling status. If quenching is done with water rather than steam, watch out for a calcium build up outboard of the seal. This “hard water” build up can restrict the movement of the flexing portion of the seal as it tries to compensate for face wear.
- If you substitute condensate for the quenching fluid the build up can be eliminated almost entirely.
Bad solution #2.
- Run a line from the discharge of the pump through a cooler and filter to cool down and clean up the oil going into the stuffing box. The problems with this solution are obvious. The filter will clog and the cooler will become inoperative as coke builds up on the tubes.
Bad solution #3.
- Use two seals and run cool oil between them. You have addressed the cooling problem but you have not addressed the problem of the slurry with this solution.
What then is the best solution that addresses all of the problems?
You should install a large jacketed sealing chamber. These bolt-on accessories are available from your local pump or seal supplier.
- Be sure to dead end the stuffing box. In other words no lines coming into or away from the inner seal chamber. Do not worry about the heat. With a six to eight gallon per minute (20 to 30 liters/ minute) flow through the cooling chamber the cooling jacket can keep the temperature down to 200° to 250° Fahrenheit (95° to 120° C) without any trouble. If you have hard water in your area condensate may be the best choice to use as the cooling medium. In some cases low pressure stream is satisfactory. If you anticipate long periods of shut down, low-pressure steam will be your best choice because it will keep the heat transfer oil at the proper low viscosity during these shut down periods.
- You should install a cartridge dual seal that has built in slurry features with the inner seal balanced in both directions. If the pump does not have precision bearings a dual motion seal with the same features will work just as well. “Two way” balance is necessary because the system and barrier fluid pressure can and will vary.
- The dual seal is necessary to conserve the expensive product and to provide a safety feature when the inboard seal wears out or fails. It will also allow you time to schedule a seal replacement.
- Install a convection tank between the two seals and use cool heat transfer oil as the barrier or buffer fluid. A lower pressure or buffer fluid is preferred. A slight pressure on the tank will allow you determine which seal has worn out or failed first. A pumping ring or forced lubrication between the seals is necessary
- Install a carbon restrictive bushing into the bottom of the stuffing box to act as a thermal barrier. Applications have worked without this bushing but not as well as with it. Any materials that have poor heat conductivity will work as well as carbon as long as they are non-sparking and dimensionally stable.
That is all there is to the application. Centrifugal force will clean up the small amount of fluid in the sealing chamber while the cooling jacket holds the temperature low enough to prevent coking and damaging the seal elastomer.
The only problem with this system is that it works so well we often forget to clean the cooling jacket on the pump. A small layer of calcium inside this jacket will act as an insulation and destroy the cooling affect of the jacket. Be sure to keep this jacket clean or substitute steam or condensate for the cooling water, and then don’t worry about it.
Here are a few additional thoughts:
- A cartridge dual bellows seal can be substituted as long as adequate vibration damping has been provided to prevent breakage of the bellows. With metal bellows seals try to rotate the fluid in the sealing chamber to prevent excessive wear of the thin bellows plates. In the past, heat treated AM350 stainless steel was the bellows material of choice. In recent years Inconel 718 is becoming popular.
- The bearing grease or lip seals should be replaced with labyrinth or positive face seals. The original equipment manufacturer (OEM) lip seals have a design life of about two thousand hours (84 days) and they will cause costly shaft fretting damage. These grease or lip seals will also allow moisture to penetrate into the bearing case dramatically reducing bearing life.
- If you eliminate these lip seals, you will be able to convert to a solid shaft and improve the “stiffness ratio” enough to prevent some of the shaft bending and vibration that is experienced at start up, and as the pump runs off of its’ best efficiency point.
- Cool oil flush with a restriction bushing installed into the bottom of the stuffing box, is another choice. Be sure that the flushing pressure remains at least one atmosphere (15 psi. or 1 bar) higher than the stuffing box pressure.
- Do not hydrostatically test the seal with water. Any moisture left in the seal or trapped in a gasket will flash to steam when the hot oil enters the seal. This could be dangerous.
- When using an API (American Petroleum Institute) type gland be sure to check that the quench and drain ports have not been confused with the flush ports. If these ports are connected incorrectly it could be very dangerous.
- If you are using stationary bellows seals with a cool oil flush be careful to direct the flushing fluid away from the seal face. Since the bellows is not rotating the cooling on one side and the hot system temperature on the other can cause the bellows seal face to go “out of flat”.
- Recent tests show that carbon faces always experience some pitting in hot oil applications. In the past these pits were ignored, but fugitive emission standards dictate that two hard faces should be use in all hot oil applications.