Selecting the Proper Boiler Feed Pump
When selecting a feed pump for an industrial steam boiler, there are a number of steps to take into account to ensure safe operation. A steam boiler is a pressurized tank, and it is critical that it always has the correct amount of water to avoid a boiler meltdown. For this and other reasons, the feed water pump has to be sized for a flow that is a certain percentage higher than what the boiler requires for normal operation.
It is also important to be cognizant of the vented receiver or deaerator water temperature and the presence of treatment chemicals, both of which can have an impact on materials of the pump and mechanical seal.
Steam boiler systems are available in a broad range of sizes. The below steps apply to pump selection for an industrial steam boiler with a horsepower range of 100 to 1200 and design pressure from 150 to 500 psig.
In large applications, it is common to install multiple boilers parallel to one another. There are two common designs for feed water pumping systems for multiple boiler applications. The first is a direct feed for multiple boilers. This type of system is very common because it is easy to maintain and switch out a complete boiler or feed pump.
The second method is known as “booster sets for feeding multiple boilers.” Instead of having individual feed pumps supply each boiler, there is a booster set that supplies water to a common manifold, from which the water moves to individual boilers through its own modulating feed water valve.
Follow these 16 steps to determine the proper boiler feed water pump for your system.
- Determine the control method to be used. Check the specification.
- Calculate the BASE flow rate. The BASE flow rate is found using the formula: boiler maximum capacity horsepower x 0.069 x C. Use the boiler maximum capacity horsepower from the boiler manufacturer’s specification. The number 0.069 is a constant. The “C” value varies depending on whether the pump will cycle on and off in intermittent operating mode (C=1.50), or if it will operate in a continuous feed mode (C=1.15).
- Add for continuous boiler blowdown flow. This step is not always required and is not always specified. When needed, it is often about 10 percent of the pump’s best efficiency point flow rate, but it can vary and is set by the water treatment company. This continuous blowdown flow is intended to remove total dissolved solids (TDS) from the boiler water and is not to be confused with the bottom blowdown of the boiler water.
- Add for by-pass flow, if required. Some control methods use a flow by-pass recirculation line and a feed valve. The by-pass flow ensures that some minimum amount of flow passes through the pump to avoid operating close to shutoff head. The by-pass recirculation flow must be sized based on the pump manufacturer’s minimum flow requirement for each pump. The amount of flow will vary, but is often based on about 10 to 20 percent of the pump’s nominal flow rate at the best efficiency point.
- Determine the total flow rate required. Calculate the sum of the base flow, by-pass flow and continuous blowdown flow (if required).
- Calculate BASE head for feed pump. When calculating total dynamic head (TDH) for all centrifugal pumps, consider all of the influencing factors on both the suction side and the discharge side of the pump. Use the following formula: BASE head in feet = maximum boiler operating pressure x 2.31 x 1.03 ÷ liquid specific gravity.
- Add all suction piping head components. Calculate and determine the gauge pressure in the deaerator tank, elevation head from tank’s waterline on the suction side to the pump’s lowest impeller, and all suction line friction losses.
- Add all discharge piping system head components (including feed valve, if required). Determine all friction losses on the discharge side of the pump, plus the elevation to the boiler’s feed water inlet relative to the pump’s discharge.
- Determine total pumping head. Calculate the sum of all head components, including the BASE head with safety factor, suction side piping system head and discharge side piping system head.
- Calculate head at boiler safety valve setting plus three percent. In addition to achieving the correct head at the required flow rate, the pump must also be capable of achieving a shutoff head (the head at zero flow) of three percent above the boiler safety valve setting. For example, if the boiler operating pressure is 130 psi, convert this to feet of head by multiplying by 2.31, then multiplying by 1.03 for the overage, and then dividing by the liquid’s specific gravity.
- Determine the boiler’s feed tank water temperature.
- Perform net positive suction head available (NPSHa) calculation. The formula is: absolute pressure in the feed water tank ± elevation of minimum water level in tank above feed pump - vapor pressure of water in feed tank - suction line friction loss = NPSHa.
- Make preliminary pump selection. Review the curves available to ensure the pump meets these conditions.
- Assure that shutoff head equals or exceeds three percent above safety valve setting.
- Check net positive suction head required (NPSHr) versus NPSHa. This calculation must be performed for every installation since no two are exactly alike.
- Ensure material compatibility. Check both chemical and temperature compatibility.
After these steps are completed, the final pump selection can be made. To learn more, visit www.cleaverbrooks.com/webinars to view a three-part educational series about pumping systems in the boiler room.
Steve Connor is an industry expert in steam- and hot water-generation. Jim Swetye is a senior technical trainer for Grundfos Pumps. Contact Steve at firstname.lastname@example.org and Jim at email@example.com.