The feed pump has one difficult job to perform: to introduce feed water into a boiler vessel through the entire steam pressure with a sufficient pace to substitute each pound of water which a boiler puts into steam in each minute for any load. If the pump is not the right one, it may cause the boiler to trip on low water cutoff, the pump to experience cavitation, or the system to waste energy for many years. The guide gives advice on how to manually calculate the main characteristics of the boiler feed pump.
Brief answer: The processes of determining the boiler feed pump size include three calculations. In the first stage, the flow rate is determined by calculating the flow rate in GPM, which is obtained from the comparison of steam production of the boiler in lb per hour to the product of 8.33 lb per gallon and 60 times 1 plus blowdown percentage times a safety factor. It could also be calculated through the equation for boiler horsepower where Q=BHP*34.5/8.337*60*1.5. This is based on empirical data that permits to use 1 GPM for each 10 BHP of the boiler output. The second value that needs to be determined is total dynamic head which is calculated like this: TDH in ft= relief valve pressure of 1.03 plus friction losses in psi times 2.31 plus static height. One can see that the value of 1.03 is given as per ASME section I requirements where it is stated that the pump pressure should exceed that of the relief valve by 3%. Finally, one can calculate brake horsepower by multiplying GPM by TDH and specific gravity and dividing by 3960 and the efficiency of the pump which is typically within 65 to 75%.
Why Feed Pump Sizing Matters
A steam boiler is a pressure vessel, and it is imperative according to Section I of ASME that it must operate with sufficient water. The feed pump does that, and it has to be sized according to the basic requirement of the code: the pump should be capable of delivering water at a pressure that is 3% higher than the maximum pressure of the safety/relief valve and should compensate for all losses in the system. The consequences of making a mistake are very much real.
- In case the pump is undersized, the boiler will trip on low-water cutoff, the production will stop and the equipment will get harmed.
- If the pump installed is oversized, there will be energy loss, short cycling, and early wear and tear of the pump as it will run far away from its efficiency point.
- If the discharge pressure is wrong, the required amount of feed water cannot be supplied to the drum due to the pressure of steam inside, and hence cannot enter the boiler.
- If the NPSH of the feed pump is insufficient, cavitation will happen which leads to pump failure in a matter of weeks or months.
The calculations for correctly sizing the feed pump consists of three steps, the details of which are given below with calculations, practical examples and an online calculator.
Boiler Feed Pump Calculator
Just enter the parameters regarding your boiler and system into the fields given, and click on the Calculate button. The tool can be used both in US units and in metric units. You may find the full formulas under the calculator.
The calculator can be used as an initial sizing tool based on the formulas listed below—make sure to verify the final choice against both the manufacturer’s pump curve and the relevant ASME codes for your area. For a full interactive reference with worked examples, the companion site boilerfeedpumpcalculation.com provides additional boiler-feed sizing tools.
Step 1: Calculate Flow Rate
The pump should produce feed water in proportion to the maximum steam generation of the boiler, taking into account blowdown and margin for swings in load. The general equation that can be applied is shown below.
Q (GPM) = [Steam production (lb/hr)] ÷ (8.33 × 60) × (1 + blowdown fraction) × safety factor
- In this equation, 8.33 denotes the mass of one gallon of water (lb), and 60 changes hours into minutes, which means the function ÷ (8.33×60) converts lb/hr of steam to GPM of equivalent water.
- Blowdown is understood as the water artificially removed to manage dissolved solids and it usually equals 5-10% of the amount of water supplied to the pump.
- Thus, the calculations should include this amount. The safety factor equals 1.25 for the operation of modulating feed in its continuous form, or 1.50 for sporadic operation.
Example: In the case where a certain boiler operates at 3450 lb/hr (100 boiler HP), its evaporation flow will be equal to 6.9 GPM in accordance with the formula. With regard to 5% blowdown and 1.5 value of the safety factor, we get Q = 6.9 × 1.05 × 1.5 = 10.9 GPM. As a rough cross-check of our calculations, one can divide boiler HP value by 10 and obtain 10 GPM as a desired value for a 100 HP boiler.
Step 2: Calculate Total Dynamic Head
Total dynamic head (TDH) encompasses all of the factors that a pump must work against.
TDH (ft) = (Relief valve setting psi × 1.03 + friction loss psi) × 2.31 + static lift (ft)
- The 1.03 factor stems from ASME Section I code that indicates a pump discharge must exceed the highest safety or relief valve setting by at least 3%, so it is a requirement that is mandatory and not just a recommendation.
The factor of 2.31 converts pressure in psi to water head in feet (1 psi = 2.31 ft of water). - Friction loss indicates pressure drop that has taken place in pipes, valves, and connectors in between the pump and the boiler, and usually is around 5-15% of discharge pressure. It should also be noted that the feedwater control valve should also be particularly accounted for in this respect as just a modulating valve could cause a pressure gain of 10 to 25 psi, therefore it makes sense to read a guide giving information about the main line water valve before setting the layout of feedwater skid.
- Static lift is the height from the pump centerline to the boiler feedwater inlet.
Example: Relief valve set at 150 psi, friction loss equals 5 psi and static lift is 15 ft then TDH = (150 × 1.03 + 5) × 2.31 + 15 = 159.5 × 2.31 + 15 = 383 ft. And if we go metric, then pressure of one bar is approximately 10.2 m of head and thus we can use a parallel formula in this way: TDH (m) = (relief bar × 1.03 + friction bar) × 10.2 + static lift (m)
Step 3: Calculate Horsepower and Motor Size
Once you’re equipped with both the values of flow and head, pump power can be found via the following simple formula:
- Water horsepower = (GPM × TDH) ÷ 3960
- Brake horsepower = Water HP ÷ Pump efficiency (which can usually be anywhere between 65% and 75%)
Let’s take a look at an example which applies to 10.9 GPM at 383 ft of TDH: water HP = 10.9 × 383 ÷ 3960 = 1.05 HP; if pump efficiency is 68%, brake HP = water HP ÷ 0.68 = 1.55 HP. Don’t forget to select the standard motor size of 2 HP, which is the next step up from the calculated value. If you want to do it using metric units, the power (kW) = Q (m³/h) × TDH (m) ÷ (367 × efficiency) and obviously the motor has to be calculated as per power plus a service factor.
The NPSH Trap: Hot Water Cavitation
One of the critical steps usually ignored by many first-time pump buyers is the proper calculation of the Net Positive Suction Head (NPSH), which is one of the main reasons behind pump failures. While NPSH available (NPSHa) measures the suction-side safety margin above water vapour pressure, the required NPSH for a pump (NPSHr) is indicated in the pump curve. NPSHa must always remain higher than NPSHr by at least 3-5 ft. The problem is in temperature; at lower-than-room temperature water can be said to have virtually no vapour pressure, while hot water (220°F), for instance, is characterized by vapor pressure, which can be approximately equal to 40 ft of head. For example, if a deaerator tank is set 5 ft above the pump with 2 ft of suction losses, the value of NPSHa will equal atmospheric height of water (33.9 ft) plus 5 ft of height of the tank and minus the head loss of 40 ft and 2 ft, resulting in a negative NPSHa value, which leads to guaranteed cavitation. The solution to this issue is to increase height of the tank (each foot will bring one foot of NPSHa), make the piping shorter, bigger, and free from partially-closing valves, cool the water when possible, or choose a pump with a lower NPSHr.
Sizing Recommendations and Pump Types
Following the determination of system flow, head, and available power, the following principles regarding proper sizing can guide in sizing a pump properly:
- Real margin is important but not excessive. The recommended margin of safety for sizing a pumping application is 10-25% of flow rate. While sizing a pump at 2x flow rate might seem safe, it takes the pump far away from its optimal efficiency point, thus causing undesirable recirculation, vibration, and seal wear in low loads. If a great variation in pump load is expected, it is better to choose a smaller pump fitted with a VFD than a bigger pump that must be throttled down.
- Match the pump to the head. Various pump types are suited to various head levels, whereby head higher than 150 ft (45 m) would require a multistage centrifugal pump, while with heads of lower values it would be possible to use a single-stage and/or a horizontal split case pump. Pressures: horizontal split case pump – up to 300 psi, vertical turbine pump – up to 600 psi, and multistage centrifugal pump – maybe 1,000 psi.
- Size the feed tank to achieve usable storage. It is customary to assume that there will be no net time wasted by the pump larger than 10 minutes.
- Never size the pump smaller than the pressure relief valve. The pressure created by the pump should meet the relief pressure required by the system plus approximately 3% (plus losses) – meaning simply sizing the pump for working pressure does not comply with other requirements drawn by boiler standards.
- Choose materials for the water. Treated feedwater suits bronze or cast iron; deaerated hot water with dissolved oxygen calls for 304 or 316 stainless materials, and mechanical seals must tolerate the actual feedwater temperature.
Boiler feedwater systems are built from the same piping and valving families that any pressurized water system uses. The discharge line needs a check valve to stop reverse flow when the pump stops, and an isolation valve for maintenance — the two valves that protect both the pump and the boiler on every feedwater skid.
The piping between the pump and the boiler carries the full discharge pressure, so it must be specified in the right material and diameter for the flow — the same sizing logic that governs stainless steel pipe sizing in process lines, where pressure, temperature, and velocity set the pipe class rather than guesswork.
FAQ
How do I calculate the size of a boiler feed pump?
It is important to first find out the total dynamic head, the power, and the flow rate of the pump. It can be calculated through the formula: (flow = Q (GPM) which is steam output divided by [ 8.33 × 60]) × [1 + blowdown)] × safety factor (1.25 in case of modulating/1.5 in case of on-off). The total dynamic head (TDH) is calculated according to the formula given here: (relief psi × 1.03 + friction psi) × 2.31 + static lift. Then the BHP is calculated because the power can be used to find the right pump available in the market.
How to calculate feed pump rate?
The required feed rate equals the maximum evaporation rate of the boiler kiosk plus blowdown and safety margin for convenience. The formulas to be used are: (feed rate = BHP × 34.5 ÷ [8.337 x 60] × SF) or (steam output = lb/hour ÷ 500, where lb means pounds and 500 refers to the 8.33 conversion factor).
How to calculate boiler pump size?
The process of determining the "size" refers to the pump curve and also it involves flow and head values obtained from the above calculations. The pump curve is obtained through the parameters of GPM and TDH, which stands for the total dynamic head (TDH).
How to calculate pump formula?
The most common calculation for pumps involves flow rate, which is expressed in gallons per minute (GPM) of steam output; pressure in towns; power in explicit break horsepower (BHP); in addition to the conversion of 1 bar measurement to meters (m).
References
- HPAC Engineering — 16 Steps to Proper Boiler-Feed-Pump Selection
- Tip Calculator — Boiler Feed Pump Calculation Guide
- Hype Calculator — Boiler Feed Pump Sizing (NPSH and ASME Notes)
- PumpCalculator — Boiler Feed Pump Flow, Head and Horsepower
- ASME Boiler and Pressure Vessel Code, Section I
Conclusion
Boiler feed pump sizing computation is essentially based on three important parameters – however, the task is complicated by multiple traps. First one is flow which can be calculated using the maximum steam output of the boiler plus blow-down and some safety margins depending on the type of control system in use i.e. 1GPM per 10 HP of boiler capacity with additional refinements via specific formula. The second one is head value which determines pressure relief valve setting together with mandatory ASME margin of 3 pct, losses due to friction and static lift expressed in terms of 2.31ft/psi. Finally, power is equal to flow and head ratio divided by efficiency, motor power being rounded according to standard values. It is very likely that a beginner will miss some pitfalls which one is likely to overlook while sizing the pump, i.e. sizing for working pressure instead of pressure relief valve pressure, ignoring losses in control valves, or not taking into consideration that some valuable NPSH figure is cut short by dozens of feet due to high troubles with feed water vapor pressure. In the end, one has to bear in mind what different mechanisms between pump and boiler these mechanisms include – check valves, isolation valves as well as pipes which will only work properly if proper information provided in this article has been understood.
