Though handling fuel oils are not necessarily "challenging," the reliable handling of fuel oils is critical for heating and transportation systems. Rotary pumps are an excellent means of loading/unloading, transfer, and circulating fuel oil. As the following table shows, application details vary widely.
Capacities: .23 to 230+ M3/H 1 to 1,000+ GPM
Pressures: 1.75 to 175 BAR 25 to 250 PSI
Viscosities: 6.6 to 1,650 cSt 30 to 7,500 SSU
Temperatures: Outdoor ambient to 120°C Outdoor ambient to 250°F
Pump configurations will also vary from application to application. For example, pump options may include relief valves, mechanical seals, steel-fitted construction, carbon bushings, and jacketing to name a few. Drives may vary as well, from direct drives to gear reducers or V-belts.
Typical dual-pump set up for transferring No. 2 fuel oil.
Slower speeds, lower pressures, and materials designed for the anticipated temperature will pay off in longer trouble-free service, even if oil properties vary from those originally anticipated. Simple do's and don'ts of oil fuel pumping include:
Do's and Don'ts
* Install the pump as close as possible to the supply tank.
* Leave working space around the pumping unit.
* Use large, short, and straight suction piping. "Short and fat" pipes are excellent.
* Install a strainer in the suction line.
* Double-check alignment after the unit is mounted and the piping is hooked up.
* Provide overpressure protection for the discharge side of the pump, either in-line or on the pump.
* Extend service life with preventive maintenance procedures such as periodic lubrication, adjustment of end clearance, and examination of internal parts.
* Obtain, read, and keep the maintenance instructions furnished with your pump.
Don'ts
* Run a pump at faster than approved speeds.
* Run a pump at higher than approved pressures.
* Run a pump at temperatures at higher than approved temperatures.
* Use extra large, extra long suction line with a suction lift.
Installing a vacuum gauge and a pressure gauge on or near the pump will also help lengthen service life. Gauges provide a window into what is happening inside the pumping system. Properly interpreted readings can give clues to the nature of many problems. Long-term readings will show gradual changes taking place within the system.
Typical large capacity No. 6 fuel oil unloading installation.
Given the availability of certain fuels at any particular time, many fuel oil systems now accommodate No. 2 through No. 6 fuel oils. Each fuel oil differs slightly in properties and these differences place unique demands on the system.
Viscosity
* No. 2 Fuel Oil. 7.7 to 11 cSt / 35 to 50 SSU. May get sluggish at temperatures below freezing because of congealing wax.
* No. 6 Fuel Oil. 22 to 1,650 cSt / 100 to 7,500 SSU in the normal handling range of 38° to 120°C / 100° to 250°F. Viscosity may vary widely depending on the source of oil, sulfur content, and blend.
* Viscosity differences between No. 2 and No. 6 fuel oil should not be a consideration as the two fuels are never intentionally mixed in the same tank.
Temperature
* No. 2 Fuel Oil. Typically handled in the range of -30° to 38°C / -20° to 100°F.
* No. 6 Fuel Oil. Normally pumped above 38°C / 100°F to facilitate handling.
* Unheated, buried tanks with two or more feet of ground cover will normally stay within a range of 4.5° to 21°C / 40° to 70°F, depending on locale and time of year. Tanks for No. 5 and No. 6 need heaters to decrease viscosity and decrease pipe friction losses.
Lift
Lift is the vertical distance from liquid level to pump port.
* No. 2 Fuel Oil. The vacuum reading at the pump should not exceed 15" Hg. under the worst conditions. The vapor pressure of No. 2 fuel oil is less than 0.0069 BAR / 0.1 PSI at ambient temperatures. If the vacuum at the pump exceeds 15" Hg. entrained air in the oil may cause capacity and noise problems. Under vacuum conditions the air will expand and the pump will not deliver its full capacity of oil.
* No. 6 Fuel Oil. Under the worst conditions (i.e., highest viscosity and lowest liquid level), position the pump and select pipe and fitting size to give vacuum readings of 15" Hg. or less. The viscosity of No. 6 Fuel Oil varies from one tank car load to the next. Some tank cars contain "light ends" which affect viscosity and may cause lift problems.
* No. 2 or No. 6 Fuel Oil. Designing a system to handle either liquid presents problems in sizing the suction link, fittings, and strainer.
Strainer
A strainer prevent solids from entering the pump.
* No. 2 Fuel Oil. Use strainers ranging from 1/16" openings down to 100 mesh (0.00055" opening).
* No. 6 Fuel Oil. Use strainers ranging 1/16" perforated baskets to No. 10 mesh (0.074" opening) liners.
* No 2 or No. 6 Fuel Oil. A screen normally used with No. 2 would cause too much pressure drop when handling the more viscous No. 6. Either use the strainer for No. 6 all the time or change baskets when changing oils.
* During the start up of a new system, a strainer is vital for making sure that pipe scale, thread chips, dirt, and other foreign particles do not get into the pump. In general, oversized strainers (e.g., 3" strainers in a 2" line) provide ample open area to reduce pressure drop and extend the time interval between cleanings. A clean strainer should not have more than a 1" to 2" Hg. drop across it. Consider cleaning it when the drop approaches 5" Hg.
Pump
* No. 2 Fuel Oil. Many rotary pumps are suitable as long as the application parameters are within the pump limits.
* No. 6 Fuel Oil. Heavy duty pumps are normally recommended. Specify pump speed, horsepower, and clearances based on the highest viscosity anticipated.
* No. 2 and No. 6 Fuel Oil. Selecting a pump to handle either a low-viscosity No. 2 or high-viscosity No. 6 oil is problematic at best. The final selection is often a compromise. One approach is to slow down the speed of the pump and provide the horsepower needed for the No. 6 oil. Then set the pump with minimum clearances so the "slip" will not be excessive when handling the No. 2. This compromise results in a serviceable pump without any compromises in either horsepower or capacity when switching between the two oils.
Overpressure Protection
Any PD pump should have overpressure protection. That is, there should be some provision in the system or drive to protect the pump and piping from excessive pressures in case of unintentional closing of the discharge line.
Some manufacturers include relief valves as standard equipment on heavy duty pumps. Moreover, a second pressure relief valve is mounted in the line set at a lower pressure than the one on the pump. This keeps the pump-mounted valve from bypassing except in extreme emergencies. Continual bypassing of the pump-mounted valve can cause heat buildup, vapor bubbles, and a potentially dangerous situation.
Mount inline pressure relief valves near the pump. Significant pressure drops may result if the valves are far away from the pump. Also bear in mind that a small return line from the valve may have a significant pressure drop which adds to the differential pressure across the valve.
Suction Line Size & Suction Check Valve Location
If the pump is below the storage tank, suction line size on either a No. 2 or No. 6 fuel oil system is not a problem as long as calculations indicate the vacuum condition at the pump will not exceed 15" Hg. Even with a "flooded" suction (a liquid above the level of the pump) it is possible to pull a vacuum at the pump if the pipe friction losses exceed the suction head pressure.
With the pump above suction lift, the liquid level in an underground storage tank, the suction line size for handling No. 2 fuel oil should at least equal the port size of the pump. Lift and line loss calculations will indicate if the next size larger is necessary.
* No. 2 Fuel Oil. Some systems install a foot valve in the end of the suction line at the bottom of the tank. Although this configuration works well, getting to the valve can be difficult. A better arrangement is to have a check valve just above the tank in a horizontal run. This keeps most of the line full and allows for easier servicing of the valve.
* No. 6 Fuel Oil. If handling with a lift, the suction pipe size will normally be larger than a standard port size. Specify oversized ports to facilitate hookup. Lift and pressure drop calculations will determine the pipe size needed to stay below the 15" Hg. vacuum maximum at the pump.
A foot valve is rarely used in a No. 6 oil system. Rather, a check valve is often installed at the pump. The heavier, more viscous No. 6 does not drain back to the tank as readily as No. 2. Placing the check at the pump keeps all of the equipment in one package. Photo 3 shows a typical pump and heater set ready to be hooked up to the system piping and utilities.
Typical duplex pump and heater set. This one built by Smith-Koch, Philadelphia.
* No. 2 and No. 6 Fuel Oil. Selecting the appropriate piping size for dual applications can be difficult. A suction line large enough to reduce the friction loss to an acceptable level for No. 6 oil may be so big as to present priming problems when handling No. 2. This is especially true when the line does not have a foot valve and the check valve is close to the pump. The large, often long, line fills with air. The pump, running at reduced speeds to handle No. 6 oil, may take several minutes to remove air and prime itself. Positive-displacement pumps, when slowed down, do not serve well as air compressors, particularly if forced to discharge into a system where the pressure is controlled by a regulator set for burner pressures. An air vent or bleed will help get rid of the air on the discharge side of the pump, but it is a nuisance if it must be used frequently.
Line size for suction piping in a system with a lift should be no more than one size larger than the port size. If calculations indicate a need for a larger pipe, lower the capacity requirements, reduce the lift, shorten the piping run, or relocate the pipe before increasing pipe size again. Finally, consider placing a check valve or foot valve in the first horizontal run above the storage tank. This will keep the suction line full and allow for better priming ability.
