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Behavior of liquids of different viscosities in a safety valve

August 30, 2025
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Introduction

Pressure relief valves are important components of pressure systems and prevent damage to the system due to the leakage of excess fluid. A decisive factor in the effectiveness of these valves is  the viscosity of the liquid     . Viscosity indicates the flow resistance of a liquid. The performance of pressure relief valves for low-viscosity liquids (such as water) and viscous liquids (such as industrial oils) varies significantly.

In this article, we     will look at the effects of liquid viscosity on relief valve    performance, common problems, and technical solutions to improve valve performance under various conditions.

1. Liquid viscosity and its importance in safety valves

Viscosity is divided into two main categories:

  • Dynamic viscosity (μ)    : the internal flow resistance of a liquid (unit: Pa·s or centipoise).

  • Kinematic viscosity (ν)    : the ratio of dynamic viscosity to density (unit: m²/s or cSt)

Why is viscosity important in pressure relief valves?

  • Valve    Reaction Speed : High viscosity fluids cause a delay in opening the valve.

  • Flow Rate (Capacity):    High viscosity reduces flow rate.

  • Pressure drop    : Thicker liquids create a greater pressure drop along the way.

  • of    clogging : Viscous fluids can accumulate in the valve.

Überdruckventil2. Behavior of liquids of different viscosities in a safety valve

a) Liquids with low viscosity (such as water and gases)

  • Typical viscosity    : 1 to 100 cP (water ≈ 1 cP at 20 °C)

  • Functional properties    :

    • Fast valve response to pressure increase

    • High flow rate

    • Low pressure all along the line

  • Symptoms    :

    • Possibility of cavitation (removal of bubbles) in case of rapid discharge

    • Pressure fluctuations in the    system

b) Medium-viscosity fluids (e.g. hydraulic oils)

  • Typical Viscosity    : 100 to 1000 cP

  • Functional properties    :

    • Relatively stable performance

    • Requires a special  design   to reduce friction.

  • Symptoms    :

    • slight delay when opening the valve

    • Sensitivity to  temperature changes   (viscosity decreases with heating)

c) Fluids with high viscosity (such as lubricants, bitumen and polymer fluids)

  • Typical viscosity    : over 1000 cP

  • Functional properties    :

    • Large delay in valve operation

    • Significant  reduction in    flow rate

    • More pressure is required to activate.

  • Symptoms    :

    • Milk deposition    and constipation

    • In some cases, additional heating is required

Überdruckventil3. Influence of viscosity on the parameters of the main safety valve

Parameters     |     Influence High Viscosity     |     Technical Solutions

4. Viscous Fluid Management Design Solutions

a) Modification of the valve geometry

  • Large bore (full-flow design):    reduces flow resistance

  • Polished contact surface    : reduces friction

  • Special design of disc and seat    : prevents stuck.

b) Auxiliary systems

  • Jacket heating    : Viscosity reduction in heat-sensitive liquids

  • Permanent drains    : prevent   the formation of deposits.

  • Powerful pneumatic motors    : compensate for the force required to open

c) Materialauswahl

  • Non-stick coatings (PTFE, DLC)

  • Springs with a higher modulus     of elasticity to overcome fluid resistance

5. Calculation of viscosity when selecting a safety valve

Flow viscosity correction formula ( based on API 520):

Q_Viskosität = K_v × Q_norm

Where:

  •     K_v : Viscosity correction factor (from standard charts)

  •     Q_standard : Valve Nominal Flow Rate for Low Viscosity Fluids

Note    : Special calculations are required for liquids with a viscosity above 500 cP.

6. Relevant provisions

  • API 520 Part 2    : Guide to Selecting Valves for Viscous Liquids

  • ASME Section VIII, Part 1    : Pressure Requirements for Non-Newtonian Fluids

  • ISO 4126-1    : Performance Criteria for Certain Liquids

7. Practical examples

Case 1: Petrochemical industry

  • Liquid    : Hydrocarbon mixture  with a viscosity of  800 cP at 50 °C

  • Problem    : Standard valves clog every month.

  • Solution    : Install heated, PTFE-lined safety valves.

  • Result    : shelf life increased to 18 months.

Case 2: Food industry

  • Liquid    : Sauce with a viscosity of 3000 cP

  • Problem    : Incomplete emptying and residues in the milk.

  • Solution    : Use diaphragm valves with CIP.

  • Result    : 95% reduction in product waste.

8. Conclusion and final recommendations

  • For liquids with a viscosity greater than 100 cP    , special valves (viscous) must be specified.

  • Operating temperature    directly affects viscosity: always take this into account when designing.

  • For sensitive systems, perform flow simulation     (CFD) before making your selection    .

  • During operation   , constant monitoring of viscosity is   required.       

Technical Note    : Always check the viscosity correction factor (Kv) in the valve manufacturer’s tables. Incorrect selection can lead to     incorrect pressure relief in an emergency, leading to serious safety risks.