The Ultimate Guide to Hydraulic Pressure Relief Valve Sizing

A pressure relief valve (PRV) is, in fact, the most crucial safety element within any circuit of hydraulics. It functions as a safety breaker to prevent catastrophic failures by limiting system pressure. But, installing an incorrect PRV size or capacity could cause excessive heat to be generated and sluggish performance even worse, the failure to shield the system in an increase in pressure.

This guide outlines the most important steps needed to properly size and select the right hydraulic PRV.

1. Understand the Two Critical Roles of a PRV

When determining the proper size for a PRV you should consider the requirements of its two primary roles:

A. Primary Function: System Protection (Maximum Pressure Limit)

The valve has to be able to safeguard every downstream part by restricting the amount of pressure at a secure setting limit 

B. Secondary Function: System Flow Control (Full Pump Flow)

In various circuits (especially fixed-displacement pumps) The PRV is utilized to divert the entire flow of the pump and return it to reservoir if the actuators are not in use or the load is in a stall. This is why sizing the the capacity of the flow becomes vital.

2. Key Sizing Parameter: Maximum System Flow Rate 

The most important aspect to consider in PRV size will be what is the maximal flow speed it has to manage, as measured in L/min or GPM

The valve should be able of passing the whole flow of hydraulic pumps at pressure of relief. When the PRV is too small the pressure drop across the valve could be excessive, causing the pressure in the system to "spike" over the set point prior to the entire flow can be released.

Calculation Step:

Q PRV = Q Pump (max) * 1.10

Always include a safety margin (typically 10 %) on top of the maximum output of your pump) to accommodate system variations and changes in fluid density and wear on the components.

3. The Relationship Between Pressure and Flow (Pressure Override)

Contrary to an ideal safe valve true hydraulic relief valve must have the pressure to rise just a little above its set point (the cracking pressure) before it is able to be able to pass through the entire flow. This is referred to as pressure override.

Sizing Implication:

1. Direct-Acting PRVs These valves rely on one, large spring to keep the poppet shut. They provide the fastest response time, however they are also equipped with the benefit of a higher pressure override due to the fact that the spring requires an enormous force increase (i.e. the high pressure increase) to expand enough to permit the entire pump flow.

   • Ideal For: Fast-response, low-flow or pilot relief applications 

2. Pilot-operated PRVs (PORVs): These valves utilize a tiny sensitive pilot valve to regulate a larger balanced main stage. They have a smaller pressure override (a more streamlined flow curve) which means that the pressure remains closer to the set-point even when the flow is reduced to full.

   • Ideal to use in: High-flow systems in which pressure stability is essential, since they can prevent extreme heat and pressure spikes.

Sizing Rules: For high-flow systems choosing a PORV that is properly rated for Q PRV above calculation will ensure that the maximum pressure of your system is not greater than the rating of your components due to an override.

4. Selection Criteria Checklist

In reviewing the manufacturer's technical information sheet be sure that the valve you select is in compliance with all five requirements:

5. The Consequence of Incorrect Sizing

Undersizing the PRV (Too Small)

If the valve's recommended flow is insufficient for the pump it will cause the pressure to increase significantly over the relief setting prior to the full flow is allowed to go through. The excessive the pressure of overriding could cause damage to seals, fittings and even the actuator or pump itself.

Oversizing the PRV (Too Large)

While it is generally safer than oversizing but an oversized valve could cause an unsteady relief operation. The valve can be noisy or unstable with low flow relief that can result in vibrations, noise and premature wear on the spool or poppet. It is recommended to select the smallest valve that is dependable and meets the QPRV requirements with a minimal pressure override.

When you take the time to calculate your needed flow rate and selecting the appropriate valve type (Direct-Acting or. Pilot-operated) in accordance with your system's flow and pressure requirements to ensure the long-term safety, efficiency and reliability of your hydraulic machineries.