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Frequently Asked Questions
What is the difference between a piston and a plunger pump?
In a piston pump the seal travels with the piston inside the cylinder. In a plunger pump a smooth plunger passes through stationary packing fixed in the casing, so the sealing element does not move. Plunger designs handle higher pressures and abrasive fluids better, and the packing can be serviced without disturbing the cylinder.
Why do they reach such high pressures?
Because the sealing is positive rather than by clearance, so there is no slip path that opens up as pressure rises. The limit is set by the mechanical strength of the components and the power available to drive them, which is a far higher ceiling than any clearance-sealed pump can reach.
What is a triplex pump?
A pump with three cylinders whose strokes are offset so their deliveries overlap. That substantially smooths the flow compared with a single cylinder, which delivers only during half its cycle, and it balances the loads on the crankshaft. Triplex is the most common arrangement in high-pressure work.
Why is the flow pulsating?
Because each cylinder delivers only during its discharge stroke and nothing during its suction stroke. Multiple offset cylinders overlap these deliveries and reduce the variation, but do not remove it, so a pulsation damper is normally fitted where smooth flow or accurate measurement is needed downstream.
What wears out first?
The check valves and the packing. Valves open and close against the full pressure differential on every stroke, so they are the highest-duty component in the pump, and packing wears against the moving plunger. Both are designed as serviceable items and both are routine consumables.
Can they run dry?
Never safely. The fluid lubricates the packing against the plunger and cools it, so dry running rapidly destroys the packing and can score the plunger. Low-pressure or dry-run protection is standard on high-pressure systems, since the damage happens within seconds rather than minutes.
What suction conditions do they need?
Better than people expect. The suction stroke must fill the cylinder completely within its duration, so a restricted or long suction line causes incomplete filling, which appears as knocking, lost flow and eventually cavitation damage to the valves. A flooded suction with generous pipework is the norm.
Are they suitable for abrasive fluids?
Plunger designs tolerate them considerably better than piston designs, and ceramic-coated plungers extend that further, which is why they are used for slurry injection and drilling service. Abrasives still attack the packing and valve seats, so life is shorter and consumable cost is a real part of the running cost.
How is flow rate controlled?
By speed, since displacement per stroke is fixed - so flow is proportional to strokes per minute. Throttling the discharge is dangerous rather than merely inefficient, because a positive-displacement pump will keep delivering against a closed valve until something bursts.
Do they need a relief valve?
Always, and it is a safety requirement rather than a refinement. A positive-displacement pump generates whatever pressure the drive can produce against a closed discharge, so a correctly sized and set relief valve, plumbed back to the suction or to a safe place, is essential on every installation.