How do you calculate NPSHa for a boiler feed pump?

NPSHa = Atmospheric Head (33.9 ft) + Static Suction Head − Vapor Pressure Head at temp − Suction Friction. NPSHa must exceed NPSHr by 3–5 ft minimum.

How do you size a boiler feedwater tank?

Tank (gal) = (BHP × 34.5) ÷ 8.337 ÷ 60 × reserve minutes × 1.5. Quick: BHP × 0.69 for 10-min reserve.

How many GPM per boiler horsepower?

1 BHP ≈ 0.103 GPM with 1.5× safety factor. Quick rule: divide BHP by 10 to get approximate GPM.

Boiler Feed Pump Calculation — Complete Sizing Guide + Free Calculator (2025)

Q: How do you calculate boiler feed pump GPM?

GPM = (BHP × 34.5) ÷ 8.337 ÷ 60 × 1.5. Quick rule: BHP ÷ 10 ≈ GPM. For a 500 BHP boiler: 52 GPM.

Q: What discharge pressure does a boiler feed pump need?

P_discharge = (Relief Valve × 1.03) + piping losses. ASME requires 3% above relief valve setting minimum.

Q: How is boiler feed pump TDH calculated?

TDH (ft) = (Discharge pressure psi × 2.31) + static head (ft) + friction losses (ft).

On This Page

Free Boiler Feed Pump Calculator
What Is a Boiler Feed Pump?
Step 1 — Flow Rate (GPM)
Step 2 — Discharge Pressure
Step 3 — Total Dynamic Head (TDH)
Step 4 — NPSH Calculation
Step 5 — Brake Horsepower & Motor
Step 6 — Feedwater Tank Sizing
Complete Worked Example — 500 BHP
Quick Reference Tables
Common Sizing Mistakes
FAQ — 10 Questions Answered

⚙️

Boiler Feed Pump Calculator

GPM · Discharge Pressure · TDH · NPSH · BHP · Tank Size — all at once

Boiler Parameters

Boiler Horsepower (BHP)

Relief Valve Setting (psi)

Blowdown Rate (%)

System Parameters

Piping Friction Loss (psi)

Static Head / Lift (ft)

Pump NPSHr from Curve (ft)

Pump Efficiency (%)

Water Temperature & Reserve

Feedwater Temperature (°F)

Tank Reserve (minutes)

Flow, Pressure & Head

Required GPM

—

gal/min · 1.5× safety factor

Discharge Pressure

—

psi · ASME 3% rule applied

Total Dynamic Head

—

feet of head

Pump BHP

—

brake horsepower

Tank, NPSH & Motor

Min. Tank Size

—

gallons · standard size

NPSHa Available

—

feet available at pump

NPSH Status

—

Motor Size

—

HP · recommended

What Is a Boiler Feed Pump?

A boiler feed pump (also called a feedwater pump) is a high-pressure pump that continuously supplies treated water to a steam boiler to replace the water converted to steam. Without a properly sized feed pump, the boiler trips on low-water safety cutoff — shutting down your process and potentially damaging the pressure vessel.

Unlike a standard pump, a boiler feed pump must overcome the boiler's full operating pressure, all piping friction losses, and any elevation difference — while delivering the exact flow rate needed to match steam demand at every load condition.

ASME Code Requirement

ASME Boiler and Pressure Vessel Code requires the boiler feed pump to supply water at a pressure at least 3% above the highest safety/relief valve setting, plus all downstream pressure losses. This is a mandatory code requirement — not a guideline.

Consequences of Wrong Sizing

Undersized pump — boiler trips on low water, production stops, vessel may be damaged at peak load
Oversized pump — excessive energy consumption, short-cycling, premature mechanical wear
Incorrect discharge pressure — feedwater cannot enter the boiler drum against steam pressure
Insufficient NPSHa — cavitation destroys the pump impeller within weeks or months

Step 1 — Calculate Flow Rate (GPM)

The pump must deliver feedwater at a rate matching the boiler's maximum steam output plus a safety margin for load spikes and condensate return surges.

Boiler Feed Pump Flow Rate — Standard Formula

GPM = (BHP × 34.5) ÷ 8.337 ÷ 60 × 1.5

BHP = Boiler Horsepower
34.5 = lbs of steam per hour per BHP (from and at 212°F)
8.337 = weight of one gallon of water in pounds
60 = converts per-hour to per-minute
1.5 = safety factor (ASME-recommended)

Quick Rule of Thumb

GPM ≈ BHP ÷ 10 — a 500 BHP boiler needs approximately 50 GPM. This already includes the safety factor and is accurate within ±5% for standard firetube boilers.

On-Off vs. Modulating Control

With on-off control (pump cycling), add 100% catch-up: GPM = evaporation rate × 2. With modulating control (continuous), add only 25%: GPM = evaporation rate × 1.25. The standard 1.5× safety factor covers most typical applications.

EXAMPLEGPM Calculation — 500 BHP Boiler

1Base evaporation rate: (500 × 34.5) ÷ 8.337 ÷ 60 = 34.5 GPM

2Apply 1.5× safety factor: 34.5 × 1.5 = 51.7 GPM → select 52 GPM pump

✓Quick check: 500 BHP ÷ 10 = 50 GPM — within 4% of formula result

Step 2 — Discharge Pressure Calculation

The pump must generate enough pressure to push feedwater into the boiler drum against the boiler's internal steam pressure, plus overcome every loss in the piping between the pump and drum.

Required Discharge Pressure — ASME Formula

P_discharge = (P_relief × 1.03) + ΔP_piping

P_relief = safety/relief valve set pressure (psi)
1.03 = ASME 3% code requirement above relief valve setting
ΔP_piping = total pressure drop across all piping, valves, and fittings (psi)

Most Common Sizing Error

A modulating feedwater control valve adds 10–25 psi of pressure drop alone. Many engineers use only 5 psi for "total piping losses" — causing the pump to fail at full load. Always calculate every component individually.

Component	Typical Pressure Drop	Notes
Non-return / check valve	3–7 psi	Required on all boiler feed lines
Feed stop valve	2–5 psi	Gate or globe valve at boiler
Modulating level control valve	10–25 psi	Modulating control systems only
Economizer	3–8 psi	Waterside pressure drop if fitted
Piping friction losses	2–15 psi	Depends on pipe length, diameter, fittings
Elevation head to drum	0.433 psi/ft	1 foot of elevation = 0.433 psi

Step 3 — Total Dynamic Head (TDH)

TDH expresses the pump's total work in feet of water column rather than psi. This is essential because pump performance curves are plotted in feet of head — you must convert your pressure requirement to TDH to select the correct pump from a manufacturer's catalog.

Total Dynamic Head Formula

TDH (ft) = (P_discharge × 2.31) + H_static + H_friction

P_discharge = required discharge pressure (psi)
2.31 = conversion: 1 psi = 2.31 feet of water column
H_static = vertical lift from pump centerline to boiler drum water level (ft)
H_friction = friction losses in suction and discharge lines expressed in feet

Boiler Pressure (psi)	TDH — Cold Water	TDH — 212°F Feedwater	Difference
50 psi	115.5 ft	120.3 ft	+4.2%
100 psi	231 ft	240.6 ft	+4.2%
150 psi	346.5 ft	360.9 ft	+4.2%
200 psi	462 ft	481.2 ft	+4.2%
300 psi	693 ft	721.8 ft	+4.2%

Step 4 — NPSH Calculation

NPSH is the most critical and most often miscalculated parameter. Insufficient NPSHa is the leading cause of boiler feed pump failure through cavitation — impeller erosion caused by vapor bubble collapse.

NPSHr (Required) — a pump design characteristic supplied by the manufacturer on the pump curve. The minimum suction head needed to avoid cavitation at rated flow.
NPSHa (Available) — a system characteristic. NPSHa must always exceed NPSHr by at least 3–5 feet to provide a safe margin.

NPSHa Calculation Formula

NPSHa = H_atm + H_static − H_vapor − H_friction_suction

H_atm = atmospheric pressure head (33.9 ft at sea level = 14.7 psia × 2.31)
H_static = static head of liquid above pump centerline (ft) — positive if tank is above pump
H_vapor = vapor pressure head of feedwater at actual operating temperature (ft)
H_friction_suction = friction losses in suction piping only (ft)

The Hot Water Trap — Most Dangerous Mistake

At 212°F, water vapor pressure equals atmospheric pressure (14.7 psia = 33.9 ft head). Using cold water vapor pressure (0.36 psia) instead shows a false NPSHa of 33+ ft when you may actually have near zero. Always calculate NPSHa at the actual operating temperature.

Water Temp (°F)	Vapor Pressure (psia)	Vapor Head (ft)	Minimum Tank Elevation
70°F	0.36	0.8 ft	Pump can be at tank level
140°F	2.89	6.7 ft	2–4 ft above pump centerline
180°F	7.51	17.4 ft	6–10 ft above pump centerline
212°F	14.70	33.9 ft	4–6 ft minimum (atmospheric tank)
250°F	29.82	68.9 ft	Pressurized deaerator required

EXAMPLENPSHa at 180°F — Tank 8 ft Above Pump

1Atmospheric pressure head at sea level: 33.9 ft

2Tank elevation above pump centerline: +8.0 ft

3Vapor pressure head at 180°F (7.51 psia × 2.31): −17.4 ft

4Suction piping friction loss: −2.0 ft

✓NPSHa = 33.9 + 8.0 − 17.4 − 2.0 = 22.5 ft — safe margin above NPSHr of 4–8 ft

Step 5 — Brake Horsepower & Motor Sizing

With GPM and TDH known, calculate the mechanical power the pump needs and select the correct motor size.

Brake Horsepower Formula

BHP = (GPM × TDH × SG) ÷ (3960 × η)

GPM = required flow rate · TDH = total dynamic head in feet
SG = specific gravity of feedwater (≈1.0 for water)
3960 = constant for water (GPM × ft per HP)
η = pump efficiency as decimal (typically 0.60–0.80)

Always select the next standard motor size above the calculated BHP. Standard NEMA motor sizes: 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP. Motors should operate at 75–100% of rated load.

Calculated BHP	Select Motor Size	Notes
Up to 1.5	2 HP	Standard smallest for boiler feed
1.6 – 2.5	3 HP
2.6 – 4.0	5 HP
4.1 – 6.5	7.5 HP
6.6 – 9.5	10 HP
9.6 – 13.5	15 HP
13.6 – 19.0	20 HP
19.1 – 27.0	30 HP
27.1 – 52.0	40–50 HP	Consider VFD above 25 HP

Step 6 — Feedwater Tank Sizing

The feedwater receiver tank provides a buffer supply to the pump. It must hold enough water to keep the boiler running during peak demand spikes and condensate return surges.

Minimum Tank Capacity Formula

Tank (gal) = (BHP × 34.5) ÷ 8.337 ÷ 60 × Reserve_min × 1.5

Reserve_min = desired reserve in minutes (10 minutes is the standard minimum)
1.5 = safety factor — tanks cannot operate fully flooded or fully empty
Quick formula: Tank (gal) ≈ BHP × 0.69 for 10-minute reserve with 1.5× safety

Boiler Size (BHP)	Calculated Min (gal)	Select Tank Size	Reserve at Full Load
100 BHP	69 gal	100 gal	~14.5 min
200 BHP	138 gal	150 gal	~10.9 min
300 BHP	207 gal	250 gal	~12.1 min
500 BHP	345 gal	400–500 gal	~11–14 min
750 BHP	518 gal	600 gal	~11.6 min
1000 BHP	690 gal	750 gal	~10.9 min

Complete Worked Example — 500 BHP Firetube Boiler

Given System Parameters

Boiler: 500 BHP

Control type: On-Off

Relief valve: 150 psi

Feedwater temp: 180°F

Piping friction: 5 psi

Tank above pump: 4 ft

Pump efficiency: 70%

Suction friction: 1.5 ft

STEP 1Flow Rate (GPM)

→Formula: (500 × 34.5) ÷ 8.337 ÷ 60 × 1.5 = 51.7 GPM → select 52 GPM pump

✓Quick check: 500 ÷ 10 = 50 GPM (within 4% of formula result)

STEP 2Discharge Pressure

1ASME 3% requirement: 150 × 1.03 = 154.5 psi

2Add piping friction (5 psi + check valve 3 psi): 154.5 + 8 = 162.5 psi required

STEP 3Total Dynamic Head (TDH)

→TDH = (162.5 × 2.31) + 4 ft static = 375.4 + 4 = 379 ft TDH

STEP 4NPSH Available

→33.9 (atm) + 4.0 (static) − 17.4 (vapor @ 180°F) − 1.5 (friction) = 19.0 ft NPSHa — adequate margin

STEP 5Brake Horsepower & Motor

→BHP = (52 × 379 × 1.0) ÷ (3960 × 0.70) = 19,708 ÷ 2,772 = 7.1 BHP → select 7.5 HP motor

STEP 6Feedwater Tank Size

→(500 × 34.5) ÷ 8.337 ÷ 60 × 10 × 1.5 = 517.5 gal → Select 500-gallon standard tank

Final Selection Summary — 500 BHP System

Pump flow: 52 GPM · Discharge pressure: 162.5 psi · TDH: 379 ft · NPSHa: 19.0 ft · Motor: 7.5 HP · Tank: 500 gal

Run Your Own Calculation

Enter any BHP, relief valve pressure, and system parameters for instant results.

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Quick Reference Tables

GPM by Boiler Horsepower

Boiler BHP	Steam Output (lbs/hr)	Base GPM (no safety)	Required GPM (×1.5)	Quick Rule (BHP÷10)
50	1,725	3.4	5.2 GPM	5.0
100	3,450	6.9	10.3 GPM	10.0
150	5,175	10.3	15.5 GPM	15.0
200	6,900	13.8	20.7 GPM	20.0
300	10,350	20.7	31.0 GPM	30.0
500	17,250	34.5	51.7 GPM	50.0
750	25,875	51.7	77.6 GPM	75.0
1000	34,500	69.0	103.5 GPM	100.0
1500	51,750	103.5	155.2 GPM	150.0
2000	69,000	138.0	207.0 GPM	200.0

Discharge Pressure by Boiler Operating Pressure (ASME 3%)

Boiler Pressure	ASME Min. Discharge	+5 psi Friction	+20 psi (Mod. Valve)	TDH at +5 psi
30 psi	30.9 psi	35.9 psi	55.9 psi	83 ft
60 psi	61.8 psi	66.8 psi	86.8 psi	154 ft
100 psi	103.0 psi	108.0 psi	128.0 psi	249 ft
150 psi	154.5 psi	159.5 psi	179.5 psi	368 ft
200 psi	206.0 psi	211.0 psi	231.0 psi	487 ft
250 psi	257.5 psi	262.5 psi	282.5 psi	606 ft

Friction Factor F at 6 ft/sec Velocity

Pipe Size (nominal)	Max GPM at 6 ft/sec	F (ft per 100 ft pipe)	Typical Boiler Range
¾"	<10 GPM	23.0	Up to 100 BHP
1"	<16 GPM	16.5	150–200 BHP
1-¼"	<28 GPM	11.9	250–300 BHP
1-½"	<38 GPM	9.78	350–400 BHP
2"	<60 GPM	6.90	500–600 BHP
2-½"	<90 GPM	5.82	750–900 BHP
3"	<140 GPM	4.51	1000–1400 BHP
4"	<240 GPM	3.21	2000+ BHP

Common Boiler Feed Pump Sizing Mistakes

❌ Using Cold Water NPSH Values for Hot Feedwater

At 212°F, vapor pressure equals atmospheric pressure (33.9 ft head). Using cold water values shows a false NPSHa of 33+ ft when you may actually have near zero. Result: immediate cavitation, impeller destruction within weeks. Always use the actual operating temperature.

❌ Underestimating Piping Pressure Drop

A modulating feedwater control valve adds 10–25 psi alone. Many engineers use only "5 psi total" for all piping losses — causing the pump to fail to push water in at full boiler load. Calculate each component individually.

❌ Skipping the 1.5× Safety Factor on GPM

The base formula gives minimum flow at steady state only. Without 1.5×, the pump cannot handle load spikes, condensate return surges, or the catch-up required after a level sag. The safety factor is an industry standard — not optional.

⚠ Selecting a Tank That Is Too Small

A tank sized without the 1.5× usable volume factor will run dry before 10 minutes because tanks cannot operate fully flooded or fully empty (float valves and level controls need headroom above and below). Always apply the safety multiplier before tank selection.

⚠ Excessive Oversizing Without Turndown Analysis

A pump 3× the required flow at minimum boiler load will short-cycle in seconds, overheat from running off the left side of its curve, and wear out prematurely. Always analyze the minimum flow condition as well as maximum, and add a minimum flow bypass or recirculation line if needed.

Boiler Feed Pump Calculation — FAQ

How do you calculate boiler feed pump GPM? +

Use the formula: GPM = (BHP × 34.5) ÷ 8.337 ÷ 60 × 1.5. The 34.5 converts boiler horsepower to lbs/hr of steam, 8.337 converts lbs to gallons, 60 converts per-hour to per-minute, and 1.5 is the safety factor. Quick rule: divide BHP by 10 for an approximate GPM (within ±5% of the full formula).

What discharge pressure does a boiler feed pump need? +

ASME code requires discharge at least 3% above the safety/relief valve setting, plus all piping losses. Formula: P_discharge = (P_relief × 1.03) + ΔP_piping. For a 150 psi relief valve with 5 psi friction: (150 × 1.03) + 5 = 159.5 psi minimum. Systems with modulating control valves may add 20–25 psi more.

What is TDH and how do I calculate it? +

TDH (Total Dynamic Head) is the pump's required work expressed in feet of water column rather than psi. You need it to read pump performance curves. Formula: TDH (ft) = (Discharge pressure psi × 2.31) + static head (ft) + friction losses (ft). The constant 2.31 converts psi to feet of water column.

Why do boiler feed pumps cavitate and how do you prevent it? +

Cavitation occurs when suction pressure drops below the feedwater vapor pressure, causing the water to flash to steam. These vapor bubbles collapse on the impeller causing rapid erosion and vibration. Prevention: elevate the feedwater tank to increase static head, verify NPSHa exceeds NPSHr by 3–5 ft minimum, keep suction piping short with large bore, and always calculate NPSHa at actual operating temperature — never at cold water values.

How high must the feedwater tank be above the pump? +

It depends entirely on feedwater temperature. For 70°F water, 1–2 ft is adequate. For 180°F feedwater, typically 6–10 ft. At 212°F, at least 4–6 ft with short large-bore suction piping. Calculate NPSHa = 33.9 + static head − vapor pressure head at temperature − suction friction losses, then verify NPSHa exceeds NPSHr by at least 3 ft.

How many GPM is one boiler horsepower? +

1 BHP = 34.5 lbs/hr of steam = 0.069 GPM base evaporation rate. With the required 1.5× safety factor, 1 BHP requires approximately 0.103 GPM of pump capacity. This is the basis of the BHP ÷ 10 = GPM rule of thumb.

Single pump or duplex system — which do I need? +

A duplex system (two 100% capacity pumps — one running, one standby) is standard for boilers over 150 BHP in commercial and industrial applications. It eliminates the single point of failure. Simplex (one pump) is only appropriate for small residential or light commercial boilers where downtime is acceptable and not costly.

How do I calculate feedwater tank size? +

Tank (gal) = (BHP × 34.5) ÷ 8.337 ÷ 60 × 10 minutes × 1.5 safety factor. The 1.5 accounts for the fact that tanks cannot operate at 100% full or 100% empty — usable volume is about 67% of total. Quick formula: BHP × 0.69 = approximate minimum tank size in gallons for a 10-minute reserve.

What is the ASME code requirement for boiler feed pump pressure? +

ASME BPVC Section I requires the boiler feed pump to have sufficient capacity to feed the boiler at a pressure at least 3% above the highest safety valve set pressure. For safety valves set at 175 psi, the pump must deliver at least 175 × 1.03 = 180.25 psi, plus all downstream pressure losses. This is a mandatory code requirement.

How do I select the right motor size for a boiler feed pump? +

Calculate brake horsepower: BHP = (GPM × TDH × SG) ÷ (3960 × pump efficiency). Then select the next larger standard NEMA motor size. Typical pump efficiency is 60–75%. Never select a motor at exactly the calculated BHP — always round up to the next standard size (2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP). Motors should run at 75–100% of their rated load for best efficiency and longest life.