Assuming the feed water pump has been sized correctly for the application with excess pump pressure taken into consideration, many boiler feed pumps face an uphill battle getting them to provide optimal service life with minimal maintenance. One must first understand that there are numerous suction conditions which must be taken into account while sizing and there are 2 standard ways to operate the pump.

 

The first and most common supply of condensate to the pump is via a “vented condensate receiver”, usually raised 4-6 feet above the pump inlet with feedwater ranging from 170-200 degrees F.

 

The next most common supply of condensate to the pump is via a deaerator which is held under pressure of 5-7PSI, mounted 15-20 feet above the pump with feedwater ranging from 230-240 degrees F. The feedwater is prevented from vaporizing (boiling) by virtue of it being held above atmospheric pressure. The tank is elevated to provide NPSH (net positive suction head). This is not to be confused with “No Pumps Sold Here”.

 

From both of these supply scenarios, the pumps are normally operated by a level control device in the boiler which turns the pump on & off, or by controlling a modulating valve on the pump discharge to allow or stop feedwater from entering the boiler.

 

Generally, in either case, the first most commonly missing item in system analysis, are the compound suction gauge and the discharge pressure gauge on the pump. This would be like purchasing a car without headlights or a speedometer, and it is virtually impossible for anyone to see what the feedwater pump is or is not doing at any particular time. The suction gauge is required to ensure the pump is operating within acceptable NPSH’r of the pump, and the discharge gauge is required to confirm the pump is operating up to design pressure.

 

In olden days, peripheral turbine pumps were the most commonly used small boiler feed pumps. They would not air lock and would run happily along for a number of years even if they cavitated continuously until the impellers simply rotted away. They did however run on very close tolerances and installers where always advised to install suction strainers so as to eliminate any pipe scale from entering the pump. With todays much more efficient multistage centrifugal pumps, the clearances are much less a problem. However, installers continue to put in suction strainers which create undue suction losses and the more likelihood of cavitation problems when in fact, small particulates will simply pass through the pump 99.9% of the time. Cavitation will unveil itself IF a compound suction gauge is installed and can verify a low NPSH condition. A solution for the centrifugal pump would be to remove the strainer element or the strainer in it’s entirety from the suction pipe. Good, laminar flow into the pump suction, particularly when handling hot water is a must, and remember, the last pipe obstruction (elbow, valve, reducer / increaser) must be installed at least 5-8 pipe diameters before entering the pump. The longer, the better.

 

Todays multistage pumps typically are mounted vertically with the mechanical seal in the uppermost portion of the pump and this can be a problem should air from cavitaion ever accumulate there. The seal will lose it’s lubricating fluid and seal failure happens repeatedly. This is overcome by using the tapped vent on the upper pump casing and installing a fixed bypass orifice which will bleed back air and water to the condensate receiver or the deaerator. This bypass orifice is a MUST INSTALL, if in fact, the pump is being run 24-7 and has the possibility of running in a deadheaded (no flow) condition. A minimum flow of cooling water must enter the pump at all times so the bypass is sized to ensure this minimum flow. Without the bypass, electrical energy from the motor will be transferred to heat energy in the water in the pump until it flashes onto steam and eventually the pump will self-destruct.

 

Now for the discharge side of things. One of the most common problems observed in feedwater discharge piping is a cheap, non-return check valve. The on-off pumping system means that high pressure water just pumped into the boiler will slam the check valve shut, once the pump turns off. If this slamming is excessive, it will create no end of problems from broken seal faces to bent shafts and blown gaskets. It must be rectified with a quality, non-slamming check valve. As well, if piping is arranged such, that from the pump, the piping goes vertical, then horizontal and then back inverted toward the floor, there has been a natural air lock formed and the use of an air release valve, suitably rated for the temperature and pressure of the system, is highly recommended on the uppermost portion of the pipework.

Hopefully, some insight has bee gained by this general discussion. It however lacks all the possible problem areas of operating boiler feedwater pumps, or for that matter, numerous problem pump applications.

 

Pump Tech has been assisting and solving a number of these issues for customers for nearly 25 years. If you have comments, questions or concerns which have not been addressed here, I invite you to call, fax or email.