Why a Positive Displacement Pump Cannot Be Throttled

Why this matters

There is one sentence that explains almost every hydraulic fault a tech will ever chase, and it is this: the pump sets the flow, and the load sets the pressure. On a positive displacement machine that is not an approximation, it is the operating principle. Each revolution sweeps a fixed volume out of the casing and the pump has no mechanism for noticing that you have restricted where it goes. So a valve you close on the discharge is not a flow command. It is a pressure command, and the machine will obey it until the relief lifts or the weakest thing in the discharge path lets go.

Techs who learned pumps on centrifugals carry a habit that is harmless there and destructive here. This card runs one identical action against both families and prints what each does.

Before you touch a valve on one of these

Never close, or partly close, a valve on the discharge of a positive displacement pump that is running. Identify the family first: gear, vane, lobe, screw, piston, plunger, progressing cavity and most diaphragm metering pumps are positive displacement. If you need less flow, slow it, restroke it, or shut it down; do not restrict it.

Before starting or restarting one, confirm the relief path is present, correctly set, and not isolated by a closed valve. A relief valve behind a closed block valve is decoration. A sibling card covers what that relief is protecting and how its set pressure is decided.

On a hydraulic system, never search for a leak with your hand, a rag or a piece of cardboard held close. A pinhole in a hose at working pressure produces a jet that penetrates skin without a visible wound and injects oil into the tissue. Fluid injection injury is a surgical emergency that presents as a trivial puncture and is routinely under-triaged, so if it happens, the injured person goes to an emergency department immediately with the words "high pressure injection injury" said out loud, along with the fluid's safety data sheet. Find leaks by looking, from a distance, at a stopped and depressurised machine.

A hydraulic accumulator stores usable energy after the motor is locked out, so isolation is not finished when the power is off. That stored-energy step is its own requirement under 29 CFR 1910.147 and it is covered in the relief-valve card.

The relationship, stated in both directions

On a centrifugal pump, the impeller adds energy to the fluid and how much fluid moves depends on how hard the system pushes back. Flow and pressure trade against each other along a curve. Restrict the discharge and the machine slides up its own curve to less flow at more head, and it does so gracefully.

On a positive displacement pump, a fixed volume leaves the casing per revolution regardless of what is downstream. Flow is set by displacement times speed. Pressure is whatever the downstream system requires to accept that flow, and the pump will generate it. There is no curve to slide along, because the pump has no way to make less flow in response to more pressure.

Say the two together and the diagnostic value appears: on a centrifugal, a pressure reading tells you about flow. On a positive displacement machine, a pressure reading tells you about the load, and it tells you nothing about flow at all.

One gate: add twenty pounds of valve resistance at the duty flow

Two machines, same duty. Both deliver 55 gpm at 40 psi into the same piping. Then someone closes a globe valve on the discharge far enough that, at 55 gpm, the valve alone would take 20 psi.

Machine A is a centrifugal. Published curve: 58 psi at shutoff, 40 psi at 55 gpm, rising smoothly in between. The system was pure friction at 40 psi and 55 gpm; adding the valve makes the system demand 60 psi at 55 gpm.

• The pump rides its curve until pump pressure equals system demand
• New operating point: 47.4 gpm at 44.6 psi
• Flow fell about 14 percent; discharge pressure rose about 12 percent
• Shaft power fell, because on a low specific speed centrifugal power falls with flow

Machine B is a gear pump. Displacement 0.050 gal per revolution at 1,200 rpm.

• Theoretical flow: 0.050 x 1,200 = 60.0 gpm
• Slip correction at the design 40 psi differential, taken from the manufacturer's volumetric efficiency of about 92 percent: -8 percent, so delivered = 55.2 gpm, call it 55
• With the valve added, the pump keeps sweeping 60.0 gpm of volume, so the system pressure climbs until it can accept that flow
• Slip correction at the new differential, because slip rises roughly in proportion to differential pressure: at about 56 psi the slip term grows to roughly 11 percent, so delivered = 53.3 gpm
• New operating point: 53.3 gpm at about 56 psi
• Flow fell about 3 percent; discharge pressure rose about 40 percent

Read the two lines next to each other. The same valve movement took a seventh of the flow off the centrifugal and a thirtieth off the gear pump, while raising the gear pump's discharge pressure more than three times as much. That is not a difference of degree. The valve is a flow control on one machine and a pressure generator on the other.

The three percent that did move was not the valve's doing

This is the part worth keeping. The gear pump's flow fell by about 3 percent, and none of that reduction was the valve removing flow. It was slip: internal leakage back through the running clearances between the gears and the casing, driven by the pressure difference across those clearances. Raising the pressure raised the leak.

Three things make slip worse, and all three are field-visible:

• Higher differential pressure. Slip rises roughly in proportion to it, which is why the correction had to be recomputed at the new operating pressure rather than reused from the design point.
• Thinner fluid. Slip rises as viscosity falls, so the same pump loses more flow on hot oil than cold, and considerably more on a low-viscosity fluid than the one it was selected for. A pump metering a solvent slips far more than the same pump on a heavy oil.
• Wear. Clearances open with hours, and a worn pump's volumetric efficiency falls. A metering pump that used to hold its rate and now runs light at the same speed and stroke has usually not lost its drive; it has lost its clearances.

That gives you a real field test. Record delivered flow at a known speed and a known differential pressure when the machine is new, and re-measure at the same speed and the same differential later. The gap between them is slip, and its growth is a direct wear measurement. Comparing against a different pressure or a different fluid temperature compares two different quantities and will tell you the pump is worn when it is not.

The three ways flow actually changes

Speed. Flow is proportional to speed, exactly, not through a curve and not through a system interaction. Halve the speed and you halve the displaced volume. This is the clean control and it is why variable speed on a positive displacement pump behaves nothing like variable speed on a centrifugal, where the system curve gets a vote.

Displacement. A variable-displacement pump changes the swept volume per revolution directly. A metering pump changes stroke length or stroke frequency. This is the control the machine was built to accept.

Diverting. Send some of the flow the pump is still making back to suction or to tank through a bypass. The pump does not know the difference and the flow at the outlet falls. What this costs is energy and heat: the diverted flow was pressurised and then dumped, and on a closed hydraulic reservoir that heat has to go somewhere. A sibling card covers why a bypass looks like flow control and is not, on a different measure.

Everything else is not a flow control. That includes the relief valve, which is a protective device and destroys the fluid it recirculates if used as a duty bypass.

What a fully closed valve does to each machine

Close the discharge completely on the centrifugal and it reaches shutoff, 58 psi, zero flow. The energy still going in becomes heat in a trapped volume, so it must not be left there, especially on a hot system where the water can flash when the valve reopens - but the machine is bounded. There is a highest pressure it can make and it is on the curve.

Close the discharge completely on the gear pump and there is no bound. The pump keeps sweeping 60.0 gpm of volume into a fixed space. Pressure rises in a fraction of a second until the relief opens, the coupling shears, the drive stalls, a casing cracks, or a hose or joint fails, and which of those happens is decided by which is weakest, not by which is cheapest to fix. On a hydraulic circuit the pressure available for that event is limited only by the drive's torque.

That asymmetry is the whole reason the safety block at the top of this card leads with the valve rather than with the pump.

How to verify you got this right

Put a gauge on the discharge and change the load, not the valve. On a positive displacement machine, discharge pressure should move with whatever the downstream circuit is doing while flow stays essentially where it was; if flow moves substantially with load at constant speed, you have either a large slip problem or a pump that is not the family you thought.

Then check the one reading that catches the dangerous case: with the pump running normally, discharge pressure should sit well below the relief setting. A machine whose normal running pressure has crept up to within a few percent of the relief set point is one fouled filter away from running on relief continuously, and a pump running on its relief is heating its own fluid with the full shaft power and no useful output.

References

• Pump manufacturer documentation for the specific unit, which owns the displacement per revolution, the volumetric efficiency at stated pressure and viscosity, and the maximum permissible discharge pressure
• 29 CFR 1910.147 for isolation and stored energy on hydraulic and mechanical systems, including accumulator discharge before service
• Safety data sheet for the hydraulic fluid or process fluid, which the injured person's emergency department needs in the case of an injection injury
• See related: What a Relief Valve on a Positive Displacement Pump Is Protecting; Why a Throttling Valve Is the Most Expensive Flow Control There Is; Common Pump Types Reference