Positive Displacement Pumps and Ancillary Equipment
(a) Relief Valve:
Need for a Relief Valve:
Positive displacement pumps deliver a constant volume of fluid with each rotation regardless of the discharge pressure. If the discharge pressure in the system exceeds the pump’s design pressure due to a blocked outlet, closed valve, or other restriction, it can cause several problems:
- Overload on Pump Components: Excessive pressure can put undue stress on the pump’s internal components like bearings, gears, or vanes, leading to premature wear and potential failure.
- Pipe Rupture: The high pressure can exceed the pressure rating of the piping system, leading to pipe bursts and potential safety hazards.
- Pump Damage: In extreme cases, very high pressure can cause permanent damage to the pump housing or internal components.
Placement of Relief Valve:
To prevent these issues, a relief valve is installed on the discharge line of a positive displacement pump. This valve acts as a safety mechanism by:
- Sensing Pressure: The relief valve continuously monitors the pressure in the discharge line.
- Pressure Relief: When the pressure exceeds a pre-set level (cracking pressure), the relief valve opens, bypassing a portion of the flow back to the pump inlet (internal bypass) or to a reservoir (external bypass).
- Pressure Regulation: By diverting excess flow, the relief valve helps to maintain the pressure within the system at a safe operating level.
Typical Location:
The relief valve is typically installed as close to the pump discharge port as possible to minimize the volume of the system exposed to excessive pressure in case of a pressure surge.
(b) Pulsation Damper:
Pulsation Damper Function:
Positive displacement pumps, due to their operating principle, can generate pulsating flow. This means the flow rate is not constant but varies with each rotation of the pump’s internal element (gear, vane, piston, etc.). These pulsations can cause several problems in the piping system:
- Vibration and Noise: The pulsating flow can induce vibration in pipes and connected equipment, leading to increased noise levels and potential fatigue failure in components.
- Pressure Spikes and Drops: The rapid changes in flow rate can translate to pressure spikes and drops within the system, stressing components and potentially affecting the performance of pressure-sensitive equipment.
Pulsation Damper Operation:
A pulsation damper is a device installed on the discharge line of a positive displacement pump to mitigate these pulsations. It functions similarly to a shock absorber in a car:
- Bladder or Diaphragm: The pulsation damper typically contains a flexible bladder or diaphragm that separates two chambers – a gas chamber pre-charged with an inert gas (e.g., nitrogen) and a fluid chamber connected to the discharge line.
- Energy Absorption: During the high-flow phase of the pump cycle, the pulsating pressure in the fluid chamber compresses the gas in the gas chamber. The gas acts as a spring, absorbing the excess energy of the pulsating flow.
- Energy Release: During the low-flow phase of the pump cycle, the compressed gas in the gas chamber expands, pushing fluid back into the discharge line and smoothing out the pulsations in the flow rate.
Overall Effect: By absorbing and releasing the pulsating pressure, the pulsation damper helps to create a more steady and consistent flow of fluid within the system. This reduces vibration, noise, and pressure fluctuations, protecting components and improving overall system performance.
Note: The selection and sizing of a relief valve and pulsation damper depend on the specific pump characteristics, system pressure rating, and desired performance outcomes.