Know-how back pressure regulators

Application

Back pressure regulators control an adjustable constant pressure upstream of the valve. A spring keeps the valve close. As the inlet pressure rises the valve opens.

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 leaflet Calculation of Pressure Regulators). Select a valve whose K_vs value is 30 % greater than the calculated Kv figure. Additional allowances must be made for high-viscosity liquids or liquids which vaporise when depressurised.
Overflow valves should not be overdimensioned. Their optimum working range is within 10 % to 70 % of their K_vs value.

Selection of nominal pressure and material

The nominal pressure stage must be higher than the maximum system pressure – without safety margins. Take into account the influence of temperature (see DIN 2401-1/DIN EN 1333).

The right material is selected with consideration of the operating conditions and the properties of the medium in the customer's plant. Plant operators must check the media compatibility of their plant components; special materials such as duplex, super duplex, Hastelloy®, or titanium may need to be selected.

For corrosion resistance see Corrosion

Selecting the setting range

For good control accuracy you should select a setting range which places the required inlet pressure near its upper limit. If, for example, the controlled inlet pressure is to be 2.3 bar, you should select the 0.8 to 2.5 bar setting range, not 2 to 5 bar. If the available setting range is not wide enough you may go below the bottom limit of the setting range provided that the valve loading is kept low and a high control accuracy not required.

Selecting elastomer materials

You should select eleastomers 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 up to 2 bar: 2 - 10 m/s
Gases above 2 bar: 5 - 40 m/s

Sense line (control line)

You should install a sense line if the selected overflow valve is designed for sense line operation. The sense line should be connected at a distance of not less than 10 times nominal diameter upstream of the pressure reducing valve. No isolating valves should be installed in the sense line to avoid an excessive pressure differential between valve body and diaphragm.
To attenuate any oscillations occurring in the pipeline system, the sense line may be fitted with a restrictor which must never be fully closed during operation.
In the case of steam and liquids the sense line must be installed so as to fall towards the valve. Under special operating conditions, for example intermittent operation with dry steam, an compensation vessel must be installed. The sense line must be rigid as elastic hoses can induce oscillations.

Protecting the overflow valve

To protect the overflow valve against damage from solid particles carried in the pipeline, a strainer or filter should be fitted and serviced at regular intervals.

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 theleakage 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.

Cut-off

For the purpose of installation, servicing and isolation of the valve, shut-off valves should be installed upstream and downstream of the overflow valve. When closing the shut-off valves the upstream valve must always be closed first. A bypass line may be necessary to maintain 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.

Leakage line

If toxic or hazardous media are used the valve must feature a sealed spring cap (including setting spindle seal) fitted with a leakage line connection. When the overflow valve is installed on site a leakage line must be fitted capable of safely and pressureless draining the escaping medium in case the control valve should become defective.

Mounting position

For gases a pressure reducing valve can normally be fitted in horizontal pipelines with the spring cap at the bottom or at the top. Installation in vertical pipe runs is possible but can result in increased wear and loss of control accuracy owing to increased friction.

In the case of liquids a overflow valve should be installed with the spring cover at the bottom. Thus gas traps upstream of the valve are avoided which would cause the valve to oscillate.

For steam a overflow valve should likewise be installed with its spring cover at the bottom to protect the diaphragm against overheating by means of a layer of condensate. In case the valve must be emptied completely during operation (angle valves), it must be installed with the spring cap pointing upwards.

Start-up

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

Steam operation

If a overflow valve is installed in a steam plant the diaphragm water reservoir must be filled before the plant is started up. There must be no danger of overheating at the installation site caused by excessive ambient temperatures or insufficient heat dissipation. Overflow valve must not be insolated. In some cases an insulating of the body is permitted, but only with cast bodies. Never insulate diaphragm housing, mid section and spring cap (or open springs). Overheating caused by insulating destroys the elastomere of the control unit.

Setting the pressure

Pressure reducing valves are normally supplied by us with a relaxed spring. This means that a valve is set at the factory to the minimuminlet pressure. The required pressure should be set under operating conditions.

Maintenance

Back pressure regulators must be cleaned and serviced regularly.