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Key Specification Points for Turbine Driven Boiler Feed Water Pumps Used in Super Critical and Ultra Super Critical Coal Fired Power Plants
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By Mr. Ed Simmons PE Shaw Power Division
The Risks • The Boiler Feed Water Pump is the heart of the power plant. Design errors can lead to:
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– Reduced plant reliability – Reduced plant efficiency – Premature equipment failures
Boiler Feed Water Pump Train • When specifying or designing a boiler feed water pump train, the engineers are not just designing a pump.
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• Great care must be taken to ensure the pump, turbine driver, lube oil system, steam condensing system, gland sealing system, pump recirculation system, feed water system, deaerator and most importantly, the turbine driver control system all work together to support the operation of the pumping train which will in turn support the full load operation of the power plant.
Early Key Decision • If you are the Owner or Owner’s Engineer, you have one key decision to make prior to purchasing the steam turbine generator:
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• Do you purchase the boiler feed water pump turbine driver with the generating turbine?
Early Key Decision (cont.)
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• If you decide to purchase the turbine driver with the generating turbine, consider allowing your AE firm to execute the driver portion the contract. • While purchasing the “turbine package” may save capitol equipment dollars, you may end up spending those same dollars during the design development and review.
Early Key Decision (cont.)
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• If you decide not to purchase the turbine driver with the generating turbine, be sure the turbine driver portion of the AE contract accurately reflects the owners design philosophies and expectations.
Pump Components
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• Turbine Driver - supplies necessary torque to spin the pumps at the design speed. Motive steam is supplied by extraction steam from the generating turbine. • Main Pump - double cased barrel type multi-stage centrifugal pump. • Booster Pump – single or double stage, double suction centrifugal pump. Supplies required NSPH to the main pump. The booster pump takes suction from the Deaerator. • Booster Pump Speed Reducer – reduces the turbine shaft speed to the required operating speed of the booster pump. This is usually around 1800 RPM.
Pump and Turbine Arrangement • The second key decision you will need to make is the arrangement of the pump and turbine as they relate to one another. • Several typical arrangements are: – 1 X 100% or 2 X 50% booster and boiler feed water pump. Turbine is in between speed reducer/booster pump and main pump (common). Turbine Driver
Booster Pump
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Booster Pump Speed Reducer
Main pump
Pump and Turbine Arrangement (cont.) – 1 X 100% or 2 X 50% Single drive turbine. Turbine drives main pump which in turn drives booster pump. This arrangement requires an abnormally long coupled shaft to allow for the removal of the BFP cartridge. (less common)
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Turbine Driver
Booster Pump Speed Reducer
Main pump
BFP cartridge pull space
Booster Pump
Sizing The Pump
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• The AE firm will complete a pump head sizing calculation that should include the pressure required at the boiler tie point, feed water heater losses, valve and piping losses and any height differences between the pump centerline and the boiler tie point. Expect to see values in the 12,000 ft range. • The AE firm should also do the same for the interstage pump take off (IPTO). The IPTO header usually supplies the reheater attemperator with spray water. • The pump suction flow is taken from the cycle heat balance. • The pump suction flow, discharge flow and head, IPTO flow and head and desired pump speed should all be written into the pump specification. • Be sure to include this information for the start up, design and over pressure (if required) operating points for the boiler.
Key Pump Design Points • The pump specification should include:
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– Net positive suction head ratio (available to required) greater than 1.5 – provides sufficient margin to prevent cavitation – 10 – 25% rise to shut off – verifies hydraulic stability – Suction specific speed less than 9000 – ensure a moderate level of minimum flow.
Pump Curves • The pump specification should require the pump manufacturer to produce a family of curves plotted every 500 RPM. The family of curves should cover the entire speed range from the pump minimum speed through the turbine over speed trip point (usually 110% of the design speed).
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• Pump curve information should include minimum flow over the entire speed range, NPSH requirements for both the booster pump and main pump, efficiency and required input power.
Pump Margins • If not specified in the AE contract, the Owner/Owner’s engineer should work with the AE to ensure mutually acceptable margins are placed on the boiler feed water pumps. – Flow margins can range from 5 – 10% – Head margins can range from 0 – 5%
• Excessively high margins on flow and head can lead to higher design speeds.
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• Margins placed upon variable speed pumps present themselves in terms of pump speed. The “design” point (including the margins) will have a higher pump speed and the “actual” flow and head requirements of the power plant.
Pump Materials • If not specifically stated in the AE contract, the pump specification should allow the manufacturer to select the appropriate materials. • Expect to see some of the following:
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– CA6NM 12% chrome steel impeller, diffusers and other wetted components – 309 SST clad forged steel or F22 Chrome steel barrel – A463 400 series stainless steel shaft – ABSOLUTELY no plain or cast carbon steels
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Feed Water System Connections • Pump Discharge – all piping and components, including the pump barrel, up to and including the second isolation valve should be rated to the pump shutoff head at turbine overspeed. This information can be obtained directly from the pump curves. • Pump discharge piping will be A106 Gr C. • Due to the required wall thickness, the discharge piping will be custom made. Wall thicknesses up to 4” thick will be common.
Feed Water System Connections (cont.)
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• IPTO header – design pressure will equal the shutoff pressure at turbine overspeed of the pump stage from which the header is taken. • This piping will also be A106 Gr C. • The piping most likely will be scheduled pipe.
Pump Testing
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• As a minimum, the pump specification should include a witnessed curve development and NPSH test. • For the curve development test, the specification should require the manufacturer to test the pump at the “highest achievable test stand speed and power levels.” • The test should include at least 6 data points. This will ensure enough data points exist to adequately develop the pump curve shape. This test should include minimum flow, design flow and run out flow. • Test tolerances should follow the recommendations of the Hydraulic institute (HI)
Pump Testing (cont.)
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• The NPSH test should be performed at a minimum of 4 different speeds. • The suction pressure should be reduced until the onset of cavitation is achieved.
Turbine Power Rating • The turbine must have the power capacity to spin the pump at all power levels required by boiler operation. However, as the pump wears, it’s efficiency will taper off. To compensate for the lower efficiency, the turbine speed control system will speed up the pump/turbine in order to maintain the correct flow and head. This translates to a higher input power requirement from the turbine. • Once the maximum pump horse power is determined, an appropriate margin should be added and written into the turbine specification. Margins between 3 and 10% are common.
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• The margins places on the turbine should take into account the margins placed on the pump. High turbine margins should not be placed on high pump margins.
Key Interface Points
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• Steam supply for the turbine driver – The designing engineer will get the steam supply pressure and temperature from the cycle heat balance. This information should be supplied to the turbine driver manufacturer as part of the initial specification. – Turbine driver supply steam usually comes from extraction 5 off the generating turbine. – During a start up when extraction steam is not available or not in enough quantity, an auxiliary steam supply system must be provided. The auxiliary steam condition must be reasonably similar to the extraction conditions. Otherwise steam temperature mis-match conditions may occur. The turbine manufacturer should be consulted when determining the auxiliary steam conditions.
Key Interface Points (cont.) • Lube Oil System
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– The lube oil system is supplied by the turbine driver manufacturer. – The lube oil system must be correctly sized to support the turbine, main pump, booster pump, booster pump speed reducer and any other train components requiring lubrication. – The boiler feed water pump specification should include a data sheet that requests oil consumption requirements for all pump components.
Key Interface Points (cont.) • Lube Oil System (cont.)
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– The Owner/Owner’s Engineer should require the generating turbine and the boiler feed water pump train both use the same type of mineral oil. This will reduce the number of oil storage tanks and the amount of stored oil. – As a general rule, the boiler feed pump train should follow the generating turbine oil type.
Key Interface Points (cont.) • Lube Oil System (cont.)
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– When integrating lube oil pressure and temperature set points into the control system, the designing engineer should use the most limiting value for any component in the pump train. – As an example, if the turbine low oil pressure set point is 24 PSIG and decreasing and the pump low oil pressure set point is 26 PSIG and decreasing, the higher value of 26 PSIG should be programmed into the plants Distributed Control System (DCS). – Both the pump and turbine specifications should request this information on a data sheet
Key Interface Points (cont.) • Turbine to Pump Coupling
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– As part of the original design requirements, the Owner should specify what type of coupling and coupling fit should be used. This decision is usually based upon the Owner’s past experience with pumps of similar energy levels. – As part of both the pump and turbine specifications, the specifying engineer should ask for rotor thermal displacement, the direction of that displacement and the anchor point from which the displacement is measured. Without this information, the coupling manufacturer will not be able to design a coupling to meet the needs of the equipment.
Key Interface Points (cont.)
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• Turbine to Pump Coupling (cont.) – As part of both the pump and turbine specifications, the specifying engineer should require each manufacturer to provide a detailed shaft end drawing that supports the type of coupling to be used. – Once again, the coupling manufacturer will not be able to design a coupling without this information.
Key Interface Points (cont.) • Turbine to Pump Coupling (cont.)
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– Only after the coupling is designed and agreed upon by the equipment manufacturers and the designing engineer, can the final locations of the turbine and pumps be determined.
Pump Train Foundation
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• The Owner/Owner’s Engineers should require the AE firm to perform a modal analysis on the foundation. The analysis should cover the entire RPM band of the pump from zero to the turbines trip speed. • In order to perform this analysis, vibration information about the turbine and pump rotors must be gained from the manufacturers. This information should be requested on a data sheet. • Also note here the equipments final location, based upon the as designed coupling lengths, must be known.
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Additional Vibration Information • In order to ensure compatibility between all the rotating elements of the pump train, the turbine specification should require the turbine manufacture to perform a torsional analysis. This analysis will determine if the pump train has a natural frequency within the speed range. • If the torsional analysis results are unfavorable, the turbine specification should require a follow on modal analysis of the pump train. This analysis will determine if there is any excitation energy available to drive the previously determined natural frequency. This would be very rare.
Additional Vibrational Information (cont.)
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• With a high degree of certainty, the designing engineer can expect to find a turbine first critical speed at the low end of the speed range. The turbine specification should require the manufacturer to state all critical speeds through 120% of the trip speed. • The pump train’s minimum operating speed should be at least 10% higher than the first critical speed of the turbine.
Conclusion
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• The boiler feed water pump train is a highly complex assembly. The designing engineer must adequately size and specify the pumps and turbine driver so that all the individual components can work together as a pumping system.
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QUESTIONS
