01
How the liquid ends differ
A packed plunger moves directly through the process liquid and uses packing to control leakage along the reciprocating surface. A diaphragm separates the process liquid from the drive. Hydraulically actuated designs use an intermediate hydraulic fluid to transmit motion and support the diaphragm.
- Plunger: process-wetted reciprocating plunger and packing
- Diaphragm: sealed process chamber
- Hydraulic diaphragm: hydraulically balanced diaphragm with replenishment and relief features
02
Where plunger pumps fit
Plunger pumps are often considered for clean, compatible liquids and high discharge pressures. They are mechanically direct and serviceable, but packing wear, adjustment and leakage management form part of normal ownership. They should not be treated as a zero-leakage design.
- High-pressure compatible-fluid duties
- Applications where controlled packing leakage is acceptable
- Sites equipped to inspect and maintain packing
03
Where diaphragm pumps fit
Diaphragm pumps are favoured where the chemical is hazardous, toxic, valuable or environmentally sensitive. The sealed liquid end reduces external leakage risk. The selected diaphragm, valves and hydraulic system must still be matched to pressure, temperature, solids and chemical compatibility.
- Hazardous or toxic chemicals
- Containment-sensitive services
- Corrosive fluids requiring non-metallic liquid ends
- Applications requiring diaphragm monitoring or double-diaphragm options
04
A practical decision rule
Start with the containment requirement. If external leakage is unacceptable, evaluate a diaphragm liquid end first. If the liquid is compatible, leakage can be safely managed and the pressure duty favours a packed design, evaluate the plunger option. Then compare accuracy, turndown, maintenance access and total lifecycle cost at the actual operating point.
- Confirm maximum differential pressure
- Review chemical and temperature compatibility
- Define acceptable leakage and detection
- Compare maintenance tasks and spares
- Verify control range at minimum flow
Frequently asked questions
Is a diaphragm pump always safer?
A sealed liquid end can improve containment, but safety depends on correct materials, relief protection, installation, monitoring and maintenance.
Can both designs meet API 675?
API 675 covers controlled-volume reciprocating pumps. Compliance depends on the complete offered design and documented project requirements, not only the liquid-end type.
Technical references
Use the current applicable standard, manufacturer manual and project specification for final engineering.
VK Pumps plunger and diaphragm metering-pump range Petrox–VK metering-pumps brochure