Series Pump Head Addition: How to calculate series pump head and avoid pressure rating issues

When a single centrifugal pump cannot generate enough head for an application, engineers have two options: use a multi-stage pump (multiple impellers on a single shaft in one casing), or install two or more pumps in series (discharge of the first pump feeds the suction of the second). Series pumping is a common solution for high-head applications, especially when existing pumps need to be repurposed or when the required head exceeds the limits of a single-stage pump. However, series operation introduces critical engineering considerations: pressure rating of casings and seals, NPSH requirements for the second pump, flow matching between pumps, and control strategies. This guide covers how to calculate series pump head, how to avoid pressure rating problems, and when to choose series pumps versus a single multi-stage pump.

How Series Pumping Works

Basic Principle

When pumps are connected in series, the discharge of the first pump (Pump A) becomes the suction of the second pump (Pump B). The flow rate through both pumps is the same (what goes in must come out), and the total head generated is the sum of the heads from each pump:

Q_total = Q_A = Q_B (same flow through both pumps)

H_total = H_A + H_B (heads add)

This is analogous to batteries in series — voltages add, current stays the same.

Series Pump Curve Construction

To construct the combined series pump curve:

1. Obtain the performance curve for each pump (head vs. flow at the same speed)
2. At each flow rate, add the heads: H_combined(Q) = H_A(Q) + H_B(Q)
3. Plot the combined curve
4. The operating point is where the combined curve intersects the system curve

Example: Two identical pumps, each 200 m³/h at 50 m head

At the BEP flow of 200 m³/h, the series combination delivers 100 m head — exactly double the single pump head.

Dissimilar Pumps in Series

Pumps in series do not need to be identical, but they must have compatible flow ranges:

• Both pumps must be able to operate at the same flow rate
• The pump with the lower BEP flow will determine the maximum efficient flow
• If one pump is significantly larger, it may force the smaller pump to operate beyond its BEP (overload)
• Best practice: use identical pumps in series for simplicity and reliability

Critical Consideration: Pressure Rating

The Pressure Cascade

In series operation, the discharge pressure of Pump A becomes the suction pressure of Pump B. This means:

• Pump A: Sees normal suction pressure (e.g., 1 bar) and generates its differential head (e.g., 50 m = 5 bar). Discharge pressure = 1 + 5 = 6 bar.
• Pump B: Sees 6 bar suction pressure (from Pump A discharge) and generates its own differential head (another 50 m = 5 bar). Discharge pressure = 6 + 5 = 11 bar.

The problem: Pump B's casing, seals, and flanges must be rated for the combined pressure (11 bar in this example), not just its own differential head. If Pump B was originally rated for 10 bar (suction 1 bar + 9 bar differential), it will be overpressurized in series service.

Pressure Rating Checklist

Worked Example: Pressure Rating Calculation

Application: Boiler feedwater, 150 m³/h, 180 m total head required

Proposed solution: Two pumps in series, each 150 m³/h at 90 m head

Pressure calculation:

• Feedwater tank pressure: 5 bar (deaerator)
• Pump A suction: 5 bar
• Pump A differential: 90 m = 9 bar
• Pump A discharge: 5 + 9 = 14 bar
• Pump B suction: 14 bar (from Pump A)
• Pump B differential: 90 m = 9 bar
• Pump B discharge: 14 + 9 = 23 bar

Pressure rating requirements:

• Pump A casing: must handle 14 bar discharge → ANSI 150# (20 bar) OK
• Pump B casing: must handle 23 bar discharge → ANSI 150# (20 bar) NOT ENOUGH! Need ANSI 300# (50 bar)
• Pump B seal: must handle 14 bar suction → standard seal (10 bar) NOT ENOUGH! Need high-pressure seal (20+ bar)
• Connecting piping: must handle 14 bar → Schedule 40 pipe OK, but verify flange rating

Common mistake: Assuming both pumps can have the same pressure rating. In series, the second pump always sees higher pressure and must be rated accordingly.

NPSH in Series Operation

NPSH for Pump B

Pump B's NPSH available (NPSHa) is determined by Pump A's discharge pressure, not by the original suction source:

NPSHa_B = (P_A_discharge / (ρ × g)) + elevation - vapor pressure - losses

Because Pump A boosts the pressure, Pump B typically has very high NPSHa — often 20–50 m. This means NPSH is usually NOT a problem for Pump B in series operation.

However: Pump A still has the original NPSH requirement and must be selected for low NPSHr. If Pump A cavitates, it reduces pressure to Pump B, potentially causing both pumps to cavitate.

NPSH for Pump A

Pump A sees the original system suction conditions. For high-temperature fluids (boiler feedwater, hot oil), NPSHa may be marginal. Select Pump A with a low-NPSHr design (larger eye, special impeller) or increase suction pressure.

Flow Matching and Control

Identical Pumps (Simplest)

When both pumps are identical, they operate at the same flow and generate the same head. Control is straightforward:

• Both pumps start simultaneously (or with a 5–10 second delay)
• Flow is controlled by a single discharge valve or VFD
• If VFD control is used, both pumps must run at the same speed (synchronized)

Dissimilar Pumps

When pumps are different, the operating point must be within both pumps' allowable flow range:

• The pump with the lower maximum flow limits the system
• The pump with the higher minimum flow limits low-flow operation
• Both pumps need minimum flow protection (individual bypass lines)
• Performance curves must be checked at the operating point

Start-Up and Shutdown Sequence

Start-up:

1. Open Pump A suction valve, fill Pump A with fluid (vent air)
2. Start Pump A, allow pressure to build
3. Open Pump B suction valve (now pressurized by Pump A), fill Pump B
4. Start Pump B
5. Open discharge valve gradually

Shutdown:

1. Close discharge valve
2. Stop Pump B first (high-pressure side)
3. Stop Pump A
4. Close suction valves (if needed for maintenance)

Critical: Never start Pump B before Pump A is running and pressurized. Starting Pump B with zero suction pressure can cause cavitation and seal damage. Never stop Pump A while Pump B is running — Pump B will lose suction pressure and may be damaged.

Minimum Flow Protection

Each pump in series needs its own minimum flow recirculation:

• Pump A minimum flow: recirculate from Pump A discharge back to suction source
• Pump B minimum flow: recirculate from Pump B discharge back to Pump B suction (NOT to the original suction source — this would short-circuit Pump A)
• Use automatic recirculation valves (ARC valves) or orifice + check valve
• Size for 25–30% of BEP flow

Series Pumps vs. Multi-Stage Pump

Comparison

When to Choose Series Pumps

• Existing pumps can be repurposed (low capital cost)
• Head requirement is moderate (100–200 m total)
• Space is available for two pumps
• Redundancy is needed (can bypass one pump for maintenance)
• Flow may vary widely (can run single pump at low demand)
• Lead time for multi-stage pump is too long

When to Choose Multi-Stage Pump

• New installation (no existing pumps to reuse)
• High head requirement (>200 m)
• Space is limited
• Energy efficiency is critical (multi-stage is more efficient)
• Critical service (API 610 BB3/BB5 for refinery/power)
• Long-term reliability is paramount
• High suction pressure (multi-stage casing designed for it)

Applications for Series Pumping

Common Applications

Series Pumping for Slurry

Slurry pumps are typically limited to 30–60 m head per stage due to wear and high specific gravity. For long-distance slurry pipelines (mining, dredging), multiple slurry pumps are installed in series along the pipeline:

• Each pump boosts pressure by 30–50 m
• 3–5 pumps in series for 150–250 m total head
• Each pump must have high-pressure casing rating (later pumps see full upstream pressure)
• Wear parts must be rated for the operating pressure
• Speed control (VFD) on each pump to balance flow and pressure

Our horizontal slurry pump range is suitable for series installation in long-distance slurry pipelines, with high-pressure casing options and wear-resistant liners.

Design Calculation Example

Problem

A chemical plant needs to transfer 120 m³/h of sulfuric acid (SG 1.84, 40°C) through a pipeline with 140 m total friction head. The suction tank is at atmospheric pressure, 2 m above pump centerline. Existing inventory has two identical 316SS centrifugal pumps, each rated 150 m³/h at 80 m head, with ANSI 150# casings (20 bar rating) and standard mechanical seals (10 bar rating).

Step 1: Calculate Required Head

• Friction head: 140 m
• Static head: 5 m (discharge elevation)
• Pressure head: 0 (both tanks atmospheric)
• Total head: 145 m

Step 2: Check Series Performance

• Each pump at 120 m³/h: ~85 m head (from curve, slightly above BEP)
• Combined head: 85 + 85 = 170 m > 145 m required ✓
• Operating point: 120 m³/h at 145 m (with throttle or VFD adjustment)

Step 3: Pressure Rating Check

• Fluid SG = 1.84 → pressure = head × SG / 10.2
• Pump A suction: 1 bar (atmospheric + 2m = 1.04 bar)
• Pump A differential: 85 m × 1.84 / 10.2 = 15.3 bar
• Pump A discharge: 1.04 + 15.3 = 16.3 bar
• Pump B suction: 16.3 bar
• Pump B differential: 85 m × 1.84 / 10.2 = 15.3 bar
• Pump B discharge: 16.3 + 15.3 = 31.6 bar

Problem:

• Pump B casing: ANSI 150# = 20 bar rating, but sees 31.6 bar → OVERPRESSURIZED!
• Pump B seal: 10 bar rating, but sees 16.3 bar suction → OVERPRESSURIZED!

Step 4: Solutions

Option A: Upgrade Pump B

• Replace Pump B casing with ANSI 300# (50 bar rating)
• Upgrade Pump B seal to high-pressure (25 bar rating)
• Upgrade Pump B flanges to ANSI 300#
• Cost: $8,000–$15,000

Option B: Use Three Pumps in Series

• Three pumps, each ~50 m head at 120 m³/h
• Pump C discharge: 1 + (50×1.84/10.2)×3 = 1 + 27 = 28 bar (still high)
• Still need high-pressure rating for later pumps
• More complex, not necessarily better

Option C: Use Multi-Stage Pump

• Single 3-stage pump, 145 m head, ANSI 300# casing
• Designed for the pressure, no rating issues
• Cost: $25,000–$40,000, but more reliable

Recommendation: Option A (upgrade Pump B) if budget is limited and existing pumps are in good condition. Option C (multi-stage pump) for a new, permanent installation.

Our multi-stage centrifugal pump range and stainless steel chemical pump range are available with high-pressure casings (ANSI 300#) and high-pressure mechanical seals, suitable for series operation. For series pump design and pressure rating verification, talk to our engineers with your fluid properties, required head, and pump specifications.

Common Problems and Solutions

Problem 1: Pump B casing or seal fails (overpressure). Pump B was not rated for the combined series pressure. Solution: calculate pressure cascade (Pump A discharge = Pump B suction). Upgrade Pump B casing, seal, and flanges to handle total pressure. Never assume both pumps can have the same rating. If Pump B cannot be upgraded, use a multi-stage pump instead.

Problem 2: Pumps don't share head equally (one does more work). Dissimilar pumps, or one pump is worn/damaged. Solution: use identical pumps for series operation. If dissimilar, verify both curves at the operating flow. Check for wear (wear ring clearance, impeller erosion) — a worn pump generates less head, forcing the other to work harder. Install pressure gauges at each pump discharge to monitor individual performance.

Problem 3: Pump B cavitates on startup. Pump B started before Pump A builds pressure, or Pump A is not running. Solution: enforce startup sequence — Pump A first, wait for pressure, then Pump B. Install interlock: Pump B cannot start unless Pump A discharge pressure is above setpoint (e.g., 5 bar). Never run Pump B with Pump A off.

Problem 4: System flow is unstable (hunting). Pumps in series can have unstable combined curves, especially if operating near shut-off. Solution: verify combined curve is stable (continuously rising to shut-off). Install minimum flow bypass for each pump. Use VFD with proper PID tuning (longer integral time). Avoid operating below 30% of BEP flow.

Problem 5: Pump A motor overloads, Pump B motor is lightly loaded. Pumps are operating at a flow where Pump A is near overload (right side of curve) and Pump B is part-load. Solution: check that both pumps are identical and operating at the same flow. If using VFDs, ensure both are synchronized to the same speed. If dissimilar, the larger pump may overload — install flow limiter or use VFD on Pump A.

Problem 6: Connecting piping between pumps fails. Piping between Pump A and Pump B sees Pump A discharge pressure, which may be higher than original piping rating. Solution: verify pipe schedule and flange rating for the connecting piping. Use the same pressure rating as Pump B suction. Include expansion joints if needed (thermal expansion). Support piping properly (pump nozzles should not carry piping weight).

Problem 7: Cannot run single pump (bypass not available). If one pump needs maintenance, the entire system shuts down. Solution: install a bypass line around each pump with isolation valves, allowing one pump to be serviced while the other runs (at reduced head). For critical service, include a full bypass with a temporary jumper pipe.

Selection Summary

1. Series operation: flow is same through both pumps, heads add (H_total = H_A + H_B)
2. Critical: Pump B sees Pump A discharge pressure as suction — must rate Pump B for total pressure
3. Calculate pressure cascade: P_B_discharge = P_suction + H_A + H_B (converted to pressure)
4. Check: casing rating, seal rating, flange rating, piping schedule for Pump B
5. NPSH is usually not a problem for Pump B (high suction pressure from Pump A)
6. Pump A must handle original NPSH conditions — select low-NPSHr design if needed
7. Startup sequence: Pump A first, then Pump B. Shutdown: Pump B first, then Pump A
8. Each pump needs individual minimum flow protection
9. Use identical pumps when possible; dissimilar pumps must have compatible flow ranges
10. Series pumps best for: retrofits, moderate head (100–200 m), redundancy needs, variable flow
11. Multi-stage pump best for: new installs, high head (>200 m), critical service, efficiency priority
12. Slurry pipelines commonly use 3–5 slurry pumps in series (each limited to 30–60 m)
13. Our multi-stage centrifugal pumps and horizontal slurry pumps support high-pressure series operation