The By-Pass Valve Is Not Optional

Pump Notes · engineering notes from Vimpo Pump

A gear pump is a positive displacement pump. It moves a fixed volume on each revolution, also when the discharge line is closed. Pressure then increases until a part fails: the coupling, the seal, the casing or the motor.

Every gear pump needs a relief path

A relief path opens when the pressure is too high and returns the flow. There are two solutions.

• Integrated by-pass valve (code BP): a relief valve on the pump cover. It returns the flow from the discharge side to the suction side inside the pump. No external pipework is necessary. Set it 1 – 2 bar above the working pressure.
• Line relief valve: a relief valve in the discharge line that returns the flow to the tank. The installer fits it.

Which series has which

• VN and VF: integrated by-pass valve standard.
• VD, VDK and VH: integrated by-pass valve as option, code BP in the model code. Without BP in the code, fit a relief valve in the discharge line.
• VD-25, VD-40, VD-50 and VDK-40, VDK-50, VDK-65: the integrated by-pass and the thermal oil jacket cannot be combined. For a heated pump of these sizes, fit a line relief valve.

A safety device, not a flow control

The integrated valve gives short-term protection. Do not use it to set the flow. If a pump runs on its by-pass for a long time, the product circulates inside the pump and becomes hot. For continuous by-pass, use a separate line valve. Use the speed, or a variable frequency drive, to set the flow.

• At high viscosity, a line relief valve is preferred to the integrated valve.
• If the pump runs in reverse, reverse the by-pass too. It always faces the suction port.

With bitumen emulsion, do not use by-pass recirculation: emulsion breaks under shear.

Before the first start

• Make sure that the relief path is open: the by-pass is set, or a line relief valve is installed.
• Do not start against a closed discharge valve.

Send us your model code or a photo of the nameplate. We tell you if your pump has an integrated by-pass.

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