Know-How differential pressure control valves

Task

Differential pressure control valves respond to a differential pressure Δp between two measuring points. In line with the application, an increasing differential pressure opens/closes the valve. The valve is self-acting without requiring any auxiliary energy or external transducers.

Valve variants

Mankenberg offers different variants, with different measuring points for the differential pressure. Each pressure reducing valve or back pressure regulator can be used as a basis for a differential pressure control valve.

Pressure reducing valve as a basis 

If the outlet pressure is to increase by the set differential pressure (spring force), the basis is a pressure reducing valve that closes as the differential pressure increases. The control pressure + spring force results in the outlet pressure.

When the line is depressurized, the valve is open and closes as soon as the set differential pressure is exceeded.

Back pressure regulator as a basis

If the inlet pressure of the valve is to increase by the set differential pressure (spring force), the basis is a back pressure regulator which opens when the differential pressure increases. The control pressure + spring force results in the inlet pressure.

When the line is depressurized, the valve is closed and opens as soon as the set differential pressure is exceeded.

One measuring point independent of the main line 

If the control chamber is not separated, the inlet or outlet pressure serves as one measuring point. The second measuring point can be independent of the main line in which the differential pressure control valve DV is installed.

Both measuring points independent of the main line

With this variant of the differential pressure control valve DV, both measuring points can be independent of the main line in which the DV is installed. The control chamber is separated.

p₁ = inlet pressure           p₂= outlet pressure          p_ST = control pressure          p_E = set pressure (difference from p_ST+ to p_ST-)

Selecting valve type and nominal diameter

Using your maximum operating data and the smallest differential pressure Δp, you should calculate the characteristic performance figure K_v (see valve calculation). Select a valve whose K_vs value is 30 % greater than the calculated K_v figure. Additional allowances must be made for high-viscosity liquids or liquids which vaporise when depressurised.

The valves should not be overdimensioned. Their optimum working range is within 10 % to 70 % of their K_vs value.

Note for valves based on a pressure reducer the reduction ratio i.e. inlet pressure p₁ divided by outlet pressure p₂. The inlet pressure acting on the cone causes the valve to open whereas the outlet pressure acting on the diaphragm/spring system causes it to close. If the reduction ratio calculated from the operating data is greater than the quoted ratio, the valve will not close.

Selecting rated pressure and valve material

The rated pressure must exceed the maximum system pressure, irrespective of safety allowances. Please note also the effect of the temperature (see DIN 2401-1/DIN EN 1333).

Selection of the setting range 

For good control efficiency, select the setting range so that your intended differential pressure is at its upper limit.

Selecting elastomer materials

You should select elastomers according to the operating temperature and the requirements of the medium. High-pressure gases, for example, can diffuse into the elastomer and cause damage when being depressurised. 

Flow velocity

Depending on pressure drop and permitted maximum noise level, we recommend the following flow velocities:

Liquids 1 - 5 m/s
Saturated steam 10 - 40 m/s
Superheated steam 15 - 60 m/s
Gases below 2 bar 2 - 10 m/s
Gases above 2 bar 5 - 40 m/s

Sense line (control line)

Allow for sense lines. Differential pressure control valves require one or two sense lines, depending on the type; connect them at a distance of 10 times the nominal size from the valve.

The sense line must be rigid as elastic hoses can induce oscillations.

Safety device

Install a safety valve in order that the max. admissible operating pressure of the valve will not be exceeded.

Protection

To protect the valve from damage caused by solid particles in the medium, a strainer or filter should be installed and serviced regularly.

Valve seat leakage

These valves are no shut-off elements ensuring a tight closing of the valve. In accordance with DIN EN 60534-4 and/or ANSI FCI 70-2 they may feature a leakage rate in closed position in compliance with the leakage classes II – V:

Leakage class II (metal sealing double seat cone) = 0.5 % K_VS value
Leakage class III (metal sealing cone) = 0.1 % K_VS value
Leakage class IV (PTFE seal cone) = 0.01 % K_VS value
Leakage class V (soft seal cone) = 1.8 x 10^-5 x Δp x D* [l/h] (*D=seat diameter)

Any low leakage requirement must be expressly specified when ordering. Valve leakage can be considerably reduced by special measures such as lapping the valve seat, using special cone seals and increasing the control (diaphragm) surfaces.

During operation, solid particles often cause damage and seat leakage.

Shut-off devices

For installation, maintenance and tight system closure, provide shut-off devices both upstream and downstream of the valve. When closing the shut-off devices, always close the valve upstream of the control valve first. A bypass line may be required for emergency operation.

Stellited seat and cone

In the case of abrasive media or liquids with pressure drops (inlet pressure minus outlet pressure) of more than 25 bar the valve cone must be stellited; for pressure drops above 150 bar the seat must be stellited as well.

Installation position

We recommend installation in a horizontal line normally with the bonnet downwards.

Start-up

Differential pressure regulating valves should be started up and operated without pressure surges, if possible. A sudden operation of upstream or downstream valves should be avoided.

Pressure setting 

The desired differential pressure can be adjusted by preloading / relieving a spring via an adjusting screw.

Differential pressure control valves are normally supplied with the spring released. The lowest differential pressure is therefore set at the factory. The adjustment of the desired set point by tensioning the spring should be done while under flow.

Maintenance

Differential pressure regulating valves must be cleaned and serviced regularly. 

ATEX

In all areas where explosive atmospheres may be present, equipment and protective systems must be selected according to the equipment group and the EPL in accordance with ATEX Directive 2014/34/EU. Mankenberg offers special ATEX versions with corresponding EU declarations of conformity. You can find more information on our ATEX pages.

Valves free of oil and grease or  silicone

Please pay attention to order an fit only spares free of oil and grease resp. free of silicone.

Please consult our engineer if extreme operating conditions apply or whenever you are in doubt.

Notes on Safety, Operating Instruction etc. MUST be followed.