Hey folks, let’s cut to the chase—if you’re running or spec-ing positive displacement (PD) pumps, you already know they’re not your average centrifugal pump. Centrifugals move fluid by spinning, but PD pumps? They trap a fixed volume of fluid each cycle and push it out—think gear pumps, diaphragm pumps, screw pumps, rotary lobe pumps, the works. But here’s the thing: a pump is only as good as its piping. I’ve seen too many ops guys slap random pipe on a PD pump and wonder why it’s seizing, cavitating, or leaking so bad their maintenance team is pulling overtime. As a PD System supplier, we’ve helped hundreds of manufacturers, food processors, and chemical plants fix bad piping setups, so let’s break down exactly what you need to make your PD pump work like it’s supposed to. Positive Displacement System
First, let’s start with the basics: suction side piping. PD pumps are way pickier about their suction conditions than centrifugals. If suction pressure is too low, or there’s too much friction in the suction line, you get what’s called “net positive suction head available” (NPSHA) issues. For centrifugals, a little NPSH shortage might just cause mild cavitation, but for PD pumps? Cavitation is a ticking time bomb—those fixed-volume chambers slam shut on vapor bubbles, and when the bubbles collapse, it erodes internal parts (gears, rotors, diaphragms) faster than a soda can in a crusher. The rule of thumb here is simple: smaller suction pressure drop = better. No tiny suction lines. No sudden elbows right at the pump inlet.
Wait, let’s clarify: the suction line should be at least the same size as the pump’s suction port. If your pump has a 2-inch suction, you don’t downsize it to 1.5 inches to save money—even though every piping book says line sizing depends on flow rate. For PD pumps, flow rate is constant (they move the same volume per revolution, no matter the pressure), so sizing for max flow is non-negotiable. We’ve seen a 2-inch rotary lobe pump with a 1.5-inch suction line lose 30% of its efficiency in 6 months because the suction restriction was starving the pump. That’s avoidable. Also, no unnecessary fittings on the suction side. Each elbow, valve, or reducer adds friction. If you have to use a reducer, make it a gradual eccentric reducer, not a concentric one. Concentric reducers can trap air in the suction line, which is a huge problem for PD pumps—air pockets mean the pump is pushing a mix of fluid and air, so you get pulsing flow, uneven pressure, and even damage to drive belts or motors. Eccentric reducers keep the line full of fluid, no air gaps—game changer.
Next, the suction side should also be free of air and vapor. A lot of operators forget PD pumps don’t self-prime like some centrifugals, but even if yours does, persistent air in the line kills performance. For pumps handling thin fluids (like solvents or light oils), you might need a larger suction line or even a suction booster if your tank is below the pump. For thick, viscous fluids (like adhesives or food batter), viscosity adds friction, so the suction line needs to be even bigger—we usually size it one full size up for fluids over 1000 cP, because thick stuff resists flowing through narrow pipes. Also, avoid long suction lines if you can. If your tank is 20 feet away from the pump, don’t run 1.5-inch pipe all that way—go 2-inch to cut down friction. We had a food processing client once that had a 25-foot suction line on a gear pump handling dough, and they were losing 40 psi just to friction. Swapping to a 3-inch line fixed that instantly—their pump pressure went up, and they didn’t have to cycle the motor as much.
Now the discharge side. This is where most people get lazy, and it causes just as many issues as bad suction piping. PD pumps generate pressure based on the system’s resistance—so if you close a valve downstream, the pressure spikes until something breaks: seals, pipes, even the pump casing. That’s why you never, ever install a fully closed valve on the discharge side of a PD pump unless it’s a special type with a built-in relief valve (more on that in a sec). The discharge line needs to be sized a little smaller than the suction line, but not too small. If your suction is 2 inches, discharge can be 1.5 or 1.75 inches, but never 1 inch—too much restriction here will make pressure surge. Also, like suction, minimize fittings here. But here’s a pro tip: add a pressure gauge right at the pump discharge. I don’t care if you have a gauge at the end of the line—you need to see exactly what pressure the pump is generating at its outlet. That lets you spot issues early: if pressure spikes out of nowhere, you’ve got a clog downstream or a relief valve that’s stuck.
The big one for discharge: relief valves. PD pumps don’t have an internal pressure limit, so without a relief valve, you’re gambling. We specify a spring-loaded pressure relief valve (PRV) on the discharge side, as close to the pump as possible. Why close? If there’s a pressure surge, the PRV opens instantly instead of waiting for the fluid to travel 50 feet to a remote valve. Also, size the PRV for the pump’s maximum flow rate. If your pump moves 50 gpm at 100 psi, your PRV needs to handle at least that—too small, and it won’t open fast enough to relieve pressure. A lot of clients try to skip the PRV because they think their system is “safe,” but we’ve had a gear pump casing rupture because a downstream line got clogged, and there was no PRV. The mess, the downtime, the replacement parts? Way more expensive than a $200 PRV. Oh, and never install a check valve between the pump and the PRV—if the check valve closes during a surge, the PRV can’t work, and that’s a disaster.
Now, let’s talk about pulsation. PD pumps produce a pulsed flow—each chamber fills and empties, so the flow line has little spikes and drops. That’s fine for some applications, but if you’re handling sensitive stuff (like food, where pulsation can cause splashing, or chemicals that need consistent dosing), it’s a problem. So what piping fixes that? Pulsation dampeners. We recommend a bladder-type dampener installed on the discharge side, right after the PRV, to smooth out those pulses. For some pumps, especially high-flow ones, you might also need a suction dampener to stabilize the inlet flow. Also, flexible joints—wait, no, not the regular kind. Use metal expansion joints (or rubber, if your fluid is compatible) to absorb vibration. PD pumps vibrate a lot more than centrifugals, especially rotary lobe or screw pumps, so vibration can loosen pipe fittings, cause leaks, or even damage the pump shaft. We always suggest 1 or 2 expansion joints near the pump inlet and outlet, depending on the pipe length.
Wait, what about valves on the discharge side? You can have valves there, just not a fully closed isolation valve. If you need to shut off the pump for maintenance, install a valve downstream of the PRV, or a bypass valve. A lot of ops guys install a gate valve right at the discharge, and then wonder why the pump locks up when they close it. That’s a classic mistake. Another thing: for pumps handling solids (like slurry pumps, or food pumps with bits of fruit or grain), you need to use piping with smooth inner walls. No corrugated pipe, no fittings with sharp edges—solids will catch on the edges and cause clogs, which lead to pressure spikes. We’ve seen a slurry pump with a corrugated discharge line clog in 2 hours, while the same pump with smooth PVC didn’t clog for 3 months.
Let’s also cover material compatibility, because that’s part of piping too. If you’re pumping corrosive chemicals, your pipe can’t be steel—use PVC, CPVC, stainless steel, or HDPE, depending on the fluid. If you’re pumping food-grade stuff, the pipe has to be FDA-approved, no galvanized steel (it leaches zinc). A lot of people pick pipe material based on price, but if it’s not compatible with your fluid, you’ll get leaks, contamination, or pipe failure. We once had a winery client who used regular steel pipe for a pump handling wine—within a month, the wine had a metallic taste, and the pipe was corroding. Swapping to stainless steel fixed that, and their product quality went back to normal.
Now, let’s address some common myths we hear all the time. Myth #1: “PD pumps can handle any piping size.” No, they can’t—small suction lines cause cavitation, too small discharge lines cause pressure surges. Myth #2: “I don’t need a PRV if I have a relief valve on my tank.” No, the tank relief is for the tank, not the pump. If the pump is generating pressure, the tank relief can’t open fast enough to stop the pump from blowing a seal or a casing. Myth #3: “Expansion joints are unnecessary.” We had a client skip them, and their pump shaft broke in 3 months because of vibration. Expansion joints aren’t a luxury—they’re a requirement for PD pump piping.
Let’s tie this all together with a real example. Last quarter, we worked with a chemical manufacturer that was having constant issues with their 3-inch gear PD pump. Their piping setup was: 1.5-inch suction line (way too small), concentric reducer, no PRV, and a 1-inch discharge line. They were having cavitation, pressure spikes, and the pump was seizing every 2 weeks. We suggested: swap to a 3-inch suction line with an eccentric reducer, add a 2-inch discharge line, install a 300-psi PRV right at the discharge, and add a 4-inch bladder dampener. The result? The pump hasn’t seized in 3 months, their efficiency went up 25%, and downtime dropped from 8 hours a week to less than 1. That’s the difference between good piping and bad piping.
So, to recap the non-negotiables for PD pump piping:
- Suction line size = same or 1 size up from pump suction port (never downsize)
- Eccentric reducers on suction side to avoid air pockets
- Gradual, minimal fittings on suction to cut friction
- Discharge line size = slightly smaller than suction, no tiny lines
- Pressure relief valve on discharge, as close to pump as possible
- No fully closed isolation valve on discharge side (use a downstream valve or bypass)
- Expansion joints to absorb vibration
- Smooth, compatible pipe material for your fluid and application
- Optional but highly recommended: pulsation dampeners for sensitive flow, and suction stabilizers for viscous/thick fluids.
Wait, one last thing: testing. Before you put the pump into full operation, do a pressure test on the piping. Run the pump at low speed first, check for leaks, check pressure readings, make sure the PRV opens at the right set pressure. We always recommend a 50 psi pressure test above your max operating pressure to catch any weak points in the pipe or fittings.
If you’re currently dealing with PD pump issues—downtime, cavitation, leaks, low efficiency—it’s almost always a piping problem. We don’t just sell pumps; we design full PD systems, so we can help you spec the right pipe, fittings, valves, and accessories to make your pump run smoothly. Don’t waste money on expensive pump repairs or replacements when a few piping tweaks can fix everything.
If you’re looking to upgrade your PD pump piping, or just need advice on getting yours right, reach out to us to connect and talk through your needs. We’re here to make sure your PD pump works as hard as you do, without the headaches.
Lubrication Pump References
- Hydraulic Institute. (2020). Positive Displacement Pump Piping Design Guidelines.
- American National Standards Institute (ANSI). (2019). Piping Systems for Process Operations.
- National Sanitation Foundation (NSF). (2021). Food-Grade Piping and Component Standards.
- Pump Manufacturers Association (PMA). (2022). Best Practices for Positive Displacement Pump Installation.
Sunshine Machinery Co., Ltd.
As one of the most professional positive displacement system manufacturers and suppliers in China, our products have good reputation in the market. Please rest assured to buy durable positive displacement system made in China here from our factory. For price consultation, contact us.
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