Pressure Reducing & Relief Valve Theory


Reducing valves are a controlling mechanism consisting of a variable orifice which is operated by pressure on a diaphragm opposing a spring load, even when this is done indirectly, as in pilot operated type of valve.

Fig 1 shows an orifice where, provided that there is an ample supply of medium on the H.P. side and an infinite volume on the L.P. side there would be a pressure reduction. The flow through the orifice shown would depend on the high pressure and low pressure and also the area of orifice, density etc, of the fluid.

Pressure Reducing Valve Theory - fig. 1
Fig. 1

Fig 2 shows the same orifice, but if a valve is moved in the direction of the arrow, the area of the orifice becomes progressively smaller, (area of the orifice now equals the circumference of the seating  opening distance of the valve lid.) and the flow would be reduced. Therefore as in Fig 3, if the valve lid is connected to a diaphragm upon which the low pressure is allowed to act it will be seen that the valve will gradually close due to the force of the high pressure on the valve lid and the force of the low pressure on the diaphragm.

Pressure Reducing Valve Theory - fig. 2 and fig. 3
Fig. 2                                                               Fig. 3

Fig 4 If we now oppose the low pressure acting upon the diaphragm with a spring we can control the opening of the valve lid and this will in turn control the low pressure. This operation is carried out in the type ‘A’ & ‘AB’ reducing valve.

Pressure Reducing Valve Theory - fig. 4
Fig. 4

Fig 5 shows a similar arrangement which is incorporated in the type ‘C’ series. In this case the high pressure acts through a nozzle and is directed against the valve lid, which is attached to the diaphragm by means of a saddle.

From diagrams Fig 4 & 5, it will be seen that if the low pressure tends to build up due to a decrease in flow, the spring will be compressed and the valve lid will move near to its seat to restore the low pressure. Conversely if the L.P. tends to decrease due to extra fluid being required the spring will relax and the valve lid will move further from its seating due to the push of the spring, and more fluid will pass to restore the L.P.

Pressure Reducing Valve Theory - fig. 5
Fig. 5

Fig 6 shows an alternative method of regulation where high pressure is balanced between the valve lid and piston. These two areas are to all intents and purposes equal and in this case the low pressure is controlled by its action on top of the valve lid against the spring acting through the piston and this is the type of operation when a more or less reasonable control is required as in the type ‘R’ and ‘W3’ reducing valve.

Pressure Reducing Valve Theory - fig. 6
Fig. 6

Fig 7 shows a more accurate type of control, as in addition to the Fig 6 arrangement there is an extra large diaphragm and piston and the low pressure acts on this large diaphragm and piston against the spring, this provides a more sensitive operation and also much tighter shut off on dead end condition.

(Note that the L.P. acts on the valve lid and under the small piston and is balanced out). This arrangement is used in type‘W1’ reducing valve.

Pressure Reducing Valve Theory - fig. 7
Fig. 7

Fig 8 this diagram shows a similar configuration to Fig 7 where the inlet pressure is balanced but in stead of using two diaphragms, lip seals are used to seal the piston. The H.P. is balanced between top of the valve lid and small piston. The reduced pressure acting on the bottom of the valve lid is balanced on top of the small piston. The L.P. is then controlled by the large piston opposed by the spring. The use of lip seals and long sensitive spring enable this type of valve to pass more liquid than a type ‘W1’ reducing valve for the same flowing pressure, when dead end pressure is the same for both types. This arrangement is used on the type ‘D’ reducing valve.

Pressure Reducing Valve Theory - fig. 8
Fig. 8

Fig 9 The diagrammatic arrangement shown Fig 9 is of the type ‘B2’ reducing valve which consists, in effect of two valve connected by ports. This arrangement enables the main valve to open to any position within its given lift with little increase or decrease in reduced pressure, because, instead of being directly operated by a diaphragm and spring, a piston is employed which is moved by pressure from the pilot valve. The pilot valve works in such a manner that as the low pressure tends to decrease it is signalled through the L.P. port to the diaphragm and as this load will be less than the spring load the pilot valve will open further and pass more steam to the piston chamber and open the main valve, conversely when the reduced pressure tends to increase the load on the diaphragm overcomes the spring load and the pilot valve closes and restricts the flow of steam to the piston chamber, and the main valve will tend to close under the influence of H.P. Theses actions maintain set reduced pressure within vary close limits.

Pressure Reducing Valve Theory - fig. 9
Fig. 9

Fig 10. Vacuum Reducing valve. Shows the arrangement of valve lid, diaphragm and spring when L.P. required is below atmospheric pressure. The action of the valve is similar to Fig 5 but in this case the spring tends to close the valve against atmospheric pressure which is trying to open it. This is how the valve is regulated-difference between atmospheric pressure and set pressure multiplied by controlling area.

Pressure Reducing Valve Theory - fig. 10
Fig. 10

Fig 11. This arrangement is similar to fig 4, but instead of having a spring to regulate reduced pressure, compressed air is used. This allows the valve lid to open and close (within limits of deflection of a diaphragm) and maintain the reduced pressure within very close limits. The air can be supplied by means of airline and regulating (reducing) valve, or from a receiver incorporated in construction of the valve. (Top hat arrangement).

Pressure Reducing Valve Theory - fig. 11
Fig. 11



The reducing valves manufactured cover a very wide range of conditions for saturated and superheated steam, compressed air, gases and liquids etc, and before despatch the valves are subjected to a hydraulic test pressure and a working test, valves can be supplied to Lloyds, A.B.S., Norske Veritas survey etc when required.

The valves are set to approximate working pressures and it is possible to that a slight adjustment will have to be made on site, especially on steam conditions where the plant and pipelines have to be warmed up.

It is important that the conditions on which the valves are to operate are given when an enquiry is received, such as temperature of superheated steam, the variation in the inlet pressure if any, and the max/min capacities in lbs per hour, cu.ft. of air per minute when on compressed air and flow in gallons per minute when used on water or other liquids. Another important point is whether the valve is required for dead end condition, as it has been found that particularly on water pressure a soft disc valve gives the best shut off when down stream stop valves are closed.

Valves are manufactured with flanged or screwed ends to suit customer requirements.

Relief valve should be fitted after the reducing valve and depending upon the conditions of service they should be adjusted to an agreed margin above the reduced pressure. Relief valves can be fitted direct to the reducing valves, except for the type ‘A’ & ‘B’ as the normal position for connecting the relief valve to a reducing valve is on the L.P. side of the reducing valve, this position on a type ‘A’ & ‘B’ would actually be on the H.P. side.

When fitting valves into new pipelines it should be ascertained that these have been thoroughly cleaned, because if there is any scale, dirt etc, in the pipes this will carried along to the first restriction in the pipeline, i.e. the reducing valve. This could cause the valve the stick and build up on dead end. It is good practice to fit a strainer in the inlet line, and should be inspected at regular intervals and cleaned otherwise it will restrict the flow of steam, air or liquid etc.

Stop valve should be fitted in the inlet and outlet pipes to isolate the reducing valve for periodic overhaul.



Should the reducing valve not work satisfactory after a period of time:-

(1) Ascertain that the pressure and capacities are exactly as order to which the valve was supplied.

(2) If the valve builds up excessively on dead end conditions check that there is no dirt or marks on the seating face.

(3) The cover of all reducing valves may be removed to expose the valve lid/seat, and the lid should move up and down slightly to see if the valve is free. A valve which is sticking may cause over compression of the spring, build up on dead end condition or on appreciable variation in reduced pressure. On valves fitted with a saddle it is important the saddle is fitted square in the body and that it does not foul against the sides of the nozzle or body as this will cause the valve to work incorrectly.

(4) If water leaks from the small hole in the spring housing/dome this indicates that seal(s)/ diaphragm(s) are worn or damaged and should be renewed. To check the seal(s)/diaphragm  on valve on air or gas service, brush a soap solution over the hole this will indicate if the seal(s)/diaphragm is leaking if bubble appears.

(5) “Chatter” usually indicates that the valve is too large for the capacity. This applies especially to valve on liquid service.

(6) Build up on dead end condition if after checking for dirt, marking on the seat face and the position of the saddle, this could mean the that the valve is to small for the capacity required and the spring is being over compressed to obtain flowing pressure and capacity.



Valve Definition

Type 3500 is a high lift right angle safety relief / relief valve. It is designed to automatically discharge a fluid or gas at a desired set pressure

(A) An automatic pressure relieving device actuated by the static pressure upstream of the valve. Characterized by a rapid opening or pop action. Normally used on Gas or vapour service. SAFETY RELIEF VALVE.

(B) An automatic pressure relieving device actuated by the static pressure upstream of the valve. And lifts in proportion to the increase in pressure above the set pressure Normally used on liquid service.  RELIEF VALVE

The three main parts in a type 3500 are:-

(1) Nozzle- which allows the fluid or gas to be piped away correctly.
(2) Disc- which opens and closes the nozzles discharge.
(3) Spring- which applies a force on the disc which opposes the force of the fluid below the disc.



When the force exerted by the pressure of the medium underneath the disc exceeds the force exerted by the spring, the disc begins to lift off the nozzle seat. The force exerted by the discharging medium on the surface of the chamber between the disc holder and the blow down ring produces a quick opening of the valve. This is due to the force and energy created by the discharging medium is much greater than the force exerted by the spring. See Fig 12.

Pressure Reducing Valve Theory - fig. 12
Fig. 12

This should be with in 10% of the set pressure for Gas and 25% for liquid. The valve will stay open until the pressure drops and the force exerted by the spring is greater than the force generated by the medium acting on the disc holder. This should be with in -7% of the set pressure for Gas and 25% for liquid.

The distance between the blowdown ring and the disc holder will have an effect on the over pressure and the blowdown pressure.

(A) The closer the blow down the ring the smaller the over pressure, the higher the blow down pressure.

(B) The further away the blowdown ring the higher the over pressure, the lower the  blowdown pressure.

(C) For liquids the blowdown ring is always set to the lowest position.



The valves range in size from a 1” x 2” with a D’ orifice to 8” x 10” with a ‘T’ Orifice. With flanges ranging from 150 to 2500 ANSI or 6 to 40 DIN with flat face, raised face or RTJ ring face.



The valve can be manufactured in various materials such as Carbon Steel, Stainless Steel, Alloy Steel, Monel and Bronze. The valves can be fitted with bellows, which perform two functions

(1) It seals off the valve guide and spring from the fluid being discharged. This is important if the medium being discharged is corrosive.

(2) It avoids variation in the set pressure in the presence of a constant or variable back pressure. Because the bellows have an effective area equal to the area of the nozzle seating face.
The valves can be fitted with a manual lifting lever. The lifting lever comes in two types.
(1) An open lever. For non hazardous fluids.

(2) A closed packed lever. For hazardous fluids or back pressures.

The valve can be fitted with a micro switch. This is to notify the operator the valve is discharging.



The valves can be used for a wide range of systems from air and steam to the Petrochemical industries.


Pressure Definitions

(1) Set Pressure
Predetermined pressure at which the valve disc starts to lift off its seat. Indicated by an audible   discharge.

(2) Reseating Pressure
Pressure at which the valve closes after full discharge.

(3) Overpressure
The increase in pressure over the set pressure to which the valve should be fully open and discharging to the required capacity. The overpressure is expressed by percentage of the set pressures. Normally 10% of the set pressure for Gas and 25% for Liquid.

(4) Working Pressure
Pressure at which the system is working, this would be below the blowdown pressure of the safety relief valve.

(5) Design Pressure
Maximum pressure allowed for a system. The safety / relief valve would not be set above this pressure.

(6) Blowdown Pressure
The difference between the set pressure and the reseat pressure. Expressed by a percentage of the set pressure. Normally 7% of the set pressure for Gas and 25% for liquid.

(7) Back Pressure
Pressure acting on the outlet side of the valve. This can be caused by the medium when the valve is discharging. (BUILD UP BACK PRESSURE) or a back pressure resulting form design of the discharge system. This can be constant or variable back pressure. (Superimposed Back Pressure).