Let’s cut to the chase: if you’re a process plant manager, maintenance lead, or piping designer who’s ever thought, “I wish I could get more flow without replacing my existing centrifugal pump,” you’ve probably circled back to parallel pump operation. It’s one of the most common, most misunderstood ways to boost system performance—and when done right, it can save you thousands on new equipment, energy costs, and downtime. I’ve spent 12 years selling centrifugal pumps to food and beverage plants, municipal water utilities, and chemical processing facilities, and I’ve seen every mistake you can make with parallel pump setups: pumps that cavitate like crazy, energy bills that double for 10% extra flow, and plants that shut down mid-shift because a parallel pump threw an impeller. Today, I’m walking you through how to operate parallel centrifugal pumps the right way, no fluff, no engineering jargon for jargon’s sake. Centrifugal Pumps
First, let’s get the basics straight. Parallel operation means running two or more centrifugal pumps side-by-side, feeding the same discharge line (or a common header) from a shared suction source. The main goal here is to increase total flow rate, while keeping the system pressure steady or adjusting it to match your process needs. Contrast that with series operation, which boosts pressure (head) for applications that need higher pressure than a single pump can provide—we’ll stick to parallel today, because that’s what most folks reach for when they need more volume.
Before you even hook up that second pump, there’s a non-negotiable pre-check that most people skip: matching the pump curves. I can’t stress this enough. Centrifugal pumps have a unique curve that plots flow rate (in gpm or m³/h) against head (pressure, in feet or meters). For parallel pumps, each individual pump’s curve needs to be nearly identical. If you pair a 100 gpm pump with a 150 gpm pump, when you run them together at your system’s average pressure, the smaller pump will only push 50 gpm—and the larger one will work way too hard, overheating, cavitating, and burning out its seal in weeks. I once had a bottling plant client come to me panicking because they paired a new pump they’d bought online with an old, beat-up pump they’d had for 10 years. The new pump’s curve was 25% off the old one, and they ended up replacing both pumps and the entire piping header within two months. Don’t do that. Ask your pump supplier (that’s me, by the way) for the exact curve specs, and make sure both pumps have the same impeller size, same motor horsepower, same maximum flow rating, and same rated head. If you can’t get identical pumps, go for pumps of the same model series, with curves that overlap by at least 90% at your system’s operating point. That’s the sweet spot.
Next up: sizing your suction and discharge piping. This is another area where corners get cut, and it’s the #1 cause of cavitation in parallel pump setups. The suction line is especially critical. When you run two pumps off the same suction header, you have to make sure the suction line is sized so that the net positive suction head available (NPSHa) at each pump’s inlet is higher than the net positive suction head required (NPSHr) for both pumps. Wait, what’s NPSH in plain English? It’s the amount of pressure at the pump inlet that keeps the liquid from boiling (which causes cavitation, the little bubbles that destroy impellers from the inside out). If your suction line is too small, both pumps have to fight for pressure at the inlet, so each pump’s NPSHa drops—boom, cavitation. For two parallel pumps, the suction header should be at least one pipe size larger than the suction inlet on each individual pump. So if each pump has a 3-inch suction flange, use a 4-inch header. Same goes for the discharge side. The discharge header should be large enough to handle the total flow of both pumps, but you also need to install isolation valves and check valves on each individual pump’s discharge line. Here’s why: if one pump shuts down for maintenance, the check valve keeps the flow from the running pump from backflowing into the idle pump, which would waste energy and cause the idle pump to rotate backwards (that’s a quick way to bend an impeller). I always tell clients to install flexible connectors on both suction and discharge lines too—parallel setups put extra vibration on piping, and flexible connectors absorb that, preventing pipe leaks and stress on the pump casings.
Now, let’s talk about the actual operation step-by-step, because even if you get the sizing and curves right, messing up the startup sequence will mess everything up. First, prepare the idle pump. For any centrifugal pump, you have to prime it before startup—fill the casing and suction line with liquid, vent all air out, because air in the lines causes loss of prime and no flow. But for parallel pumps, you need to prime each pump individually, not just the header. Then, make sure the discharge isolation valve on the idle pump is closed—wait, no, let’s adjust that: when priming the idle pump, close its discharge valve, open the vent, prime it, close the vent, then leave the discharge valve closed for now. Next, the pump you’ll run first: open its suction isolation valve, open its vent, wait for air to escape, close the vent, then slowly open its discharge valve. Gradually open the discharge valve until you reach your desired flow rate, checking the pressure gauge on the discharge header to make sure it’s steady. Now, when you’re ready to add the second pump, here’s the sequence that works every time. First, make sure the idle pump’s discharge valve is still closed. Open the idle pump’s suction isolation valve, prime it (if it’s not already primed—some setups keep both pumps primed for continuous operation), vent it, close the vent. Now, start the idle pump. Once it’s up to full speed, slowly open its discharge valve, while watching the discharge header pressure. Don’t yank it open fast—you’ll cause a pressure spike that can trip relays or burst pipes. Open it gradually until the total flow and pressure match your target. That’s it. If you have a third or fourth parallel pump, just repeat that sequence: bring each pump online one at a time, close their discharge valves, prime, start, then open their discharge valves slow.
Wait, what about trimming flow when you have parallel pumps? A lot of operators think, “If I need less flow, I just turn one pump off.” That works, but it’s not always the most efficient. Let’s say your system runs at peak flow 60% of the time, and low flow 40% of the time. If you turn one pump off for low flow, the remaining pump will have to run at a flow rate that’s well below its best efficiency point (BEP)—that’s the sweet spot where a pump runs most efficiently, with the least wear. Most centrifugal pumps have a BEP at around 70-80% of their maximum flow. If you run a pump at 30% of its BEP, it wastes 20-30% more energy, and you’ll see increased vibration and seal wear. So instead of turning one pump off, you can throttle the discharge valve on the idle pump to a small open position (just enough to keep it primed, no flow through it) or turn it on and off with a variable frequency drive (VFD). VFDs are game-changers for parallel pump operation, by the way. If you install VFDs on both pumps, you can adjust the speed of each pump independently to match your exact flow needs, instead of throttling valves. That cuts energy costs by 25-40% for most process systems, and reduces wear because you don’t have to throttle flow. I recently worked with a municipal water utility that had three parallel 500 gpm pumps running 24/7. They upgraded two pumps with VFDs, and adjusted the third to run at base speed. Now, when demand is low at night, the VFDs slow the pumps down to adjust flow, and they save $12,000 a year in electricity. That’s the kind of win that makes parallel operation worth it.
Now, common mistakes that even experienced operators make, and how to avoid them. First mistake: not accounting for system curve when setting up parallel pumps. The pump curve is only half the equation—your system has its own curve, which plots flow vs. pressure drop in the pipes, valves, and process equipment. If your system curve is steep (which means pressure drops a lot when flow increases), adding two parallel pumps will give you a much smaller flow boost than you expect. For example, if a single pump gives 100 gpm at 50 psi, and two identical parallel pumps would give 180 gpm at 50 psi, but your system needs 50 psi at 150 gpm, parallel will work. But if your system curve only allows 50 psi at 120 gpm, you’ll only get 120 gpm total, not 180. So always plot your system curve first, then match it to the combined parallel pump curve. I help all my customers do this as part of their pump design, no extra charge. Second mistake: backflow when a pump is idling. I mentioned check valves earlier, but I can’t tell you how many setups I see without them. Backflow can cause the idle pump to rotate backwards, which damages the impeller and shaft. Even if you have check valves, inspect them every six months—they get stuck closed or worn, and that’s when backflow happens. Third mistake: not monitoring NPSH. Parallel pumps increase the total flow going through the suction header, so the pressure drop in the suction line goes up when you add more pumps. That means NPSHa drops every time you bring a new pump online. Keep an eye on the suction pressure gauge when you startup a second pump—if it drops more than 10% of the required NPSHr, you need to increase the suction line size or install a booster pump on the suction side.
Another thing to consider: continuous vs. intermittent parallel operation. If you’re running parallel pumps all the time (like a municipal water system that needs 24/7 flow), you should rotate the pumps every few weeks. Run pump 1 for two weeks, then run pump 2, then pump 3, etc. That keeps all pumps evenly worn, so none of them fall behind on maintenance. If you only run parallel pumps during peak demand (like a manufacturing plant that only needs extra flow when production is high), you should run the smaller or older pump first during peak, then bring the larger or newer one online. Wait, no—actually, always have the most well-maintained pump as the lead pump, so it takes the brunt of the running time. That way, when you need to shut down a pump for maintenance, you’re not taking the only well-maintained one offline.
Now, let’s wrap this up with a quick troubleshooting guide, because even with perfect setup, things go wrong. If you add a second parallel pump and you only see a 10% flow increase instead of 80%, first check if the discharge valve on the new pump is fully open. Then check the check valve on the new pump’s discharge—maybe it’s stuck closed, so all the flow is going through the first pump. If that’s not it, check the suction line—you probably have air in the lines, or the suction size is too small, causing cavitation. If you hear a loud grinding or rattling noise, that’s cavitation—shut the new pump down immediately, check NPSHa, and adjust the suction line if needed. If you see the pressure drop when you add the new pump, that’s also a NPSH issue, or maybe the two pumps aren’t matched, so one is overpowering the other.
At the end of the day, parallel centrifugal pump operation is not rocket science, but it is detail-oriented. It works when you match pump curves, size piping correctly, follow the right startup sequence, and monitor performance regularly. If you’re thinking about adding parallel pumps to your system, or you have a setup that’s underperforming, I’m here to help. Whether you need pump curve data, piping design advice, or just someone to walk you through your startup sequence, my team can tailor a solution to your specific flow and pressure needs. No generic one-size-fits-all, no hidden fees—just expert support from someone who’s spent decades working with centrifugal pumps.
If you’re ready to chat about how parallel pumps can boost your system performance, or you need help optimizing your current setup, reach out to our team to start a conversation. We can provide custom pump recommendations, free curve matching, and on-site support if needed for your facility.
Agricultural Pump References:
- Karassik, I. J., McGuire, R. L., & Messina, J. P. (2007). Pump Handbook (4th ed.). McGraw-Hill.
- Hydraulic Institute. (2020). Pump Intake Design Standards (3rd ed.). Hydraulic Institute.
- American National Standards Institute. (2018). ANSI/HI 9.6.1 Rotodynamic Pumps for Guideline for the Proper Application of Centrifugal Pumps in Parallel.
- Grimbley, R. (2019). Parallel and Series Pump Operation: Efficiency and Best Practices. Chemical Engineering Progress, 115(5), 32-38.
Ocean Pump Co., Ltd.
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