Pressure Sustaining (BPRV)
Overview-Pressure Sustaining Valves
Pressure Sustaining or Back Pressure Regulating Valves regulate upstream pressures by relieving the surplus pressure downstream or via a return line (when installed in a tee). They work in much the same way as a relief valve (normally closed, opening at the set pressure) but are more accurate, responsive and only open far enough to relieve sufficient pressure to maintain the desired pressure upstream. The pressure to be maintained is known as the set pressure and is easily adjusted by turning the adjusting bolt, thus compressing the spring (clockwise increases the set pressure, anti-clockwise reduces the set pressure).
There are some general rules to help select the best regulator and the best control for each application:-
• Unlike other valves where the valve size matches the pipe size, these pressure control valves must be “sized”, using the differential between upstream/downstream pressures and considering whether the valve has to bypass a small percentage of the flow or the whole output of the pump. For this reason it is not unusual to select a valve that is different to the pipe size.
• Self acting valves are the simplest design and represent the lowest cost but they are not as accurate as pilot operated valves.
• Always use the lightest spring (smallest range that covers the set pressure)
• Where high pressures occur upstream and there is minimal back pressure downstream, the high differential pressure may cause cavitation. Some of the effects of cavitation can be excessive noise, premature wear of valve or other components in the downstream piping. A simple remedy is to create some back pressure (e.g. install a globe valve, partially closed) in the downstream piping.
The following range is extensive, mainly because every regulator has limitations on sizes, materials of construction, suitability to various media, inlet pressures and reduced pressure ranges. In addition to the metal bodied valves offered we have quality thermoplastic sustaining valves (DHV716/725) as well.
Selection: RELIEF or SUSTAINING?
Pressure Relief Valve (pictured left)
Is designed for relief of excess pressure, usually 10-15% above the system operating pressure. Its main purpose is to protect the pressure vessel, pump, pipes, valves and other equipment from damage by excess pressure. Once the set pressure is reached, the valve opens to evacuate the media and will reseat again once the pressure has been relieved. It is recommended that the valve is inspected after each blow-off to ensure correct re-seating and operation. In many designs, the discharge port is at right angles to the inlet and is often larger than the inlet size, thus assisting rapid discharge of the product.
If there is a need for frequent operation, this is not the correct valve to use and a back pressure valve (as pictured below) is the better selection.
Safety Valves used on compressible media (steam, air etc) often have a specially designed disc profile to assist in rapid valve lift and less overpressure. They usually have a lifting lever to manually lift the disc, thus confirming operation and to assist clearing debris or scale from the seat and disc.
Correct sizing is essential for proper operation.
Pressure Sustaining (or Back Pressure) Valve (Jordan Mk50 & JBPH pictured above)
Can be used in several ways and is commonly used to maintain the upstream pressure in a ring main or piping system. In a typical installation, it is installed in a “tee” off the main line where it remains closed until the set pressure is reached. Although it opens at a predetermined setting like the relief valve, it does not necessarily discharge at full capacity. Rather, it only opens far enough to relieve the excess pressure and can modulate and operate 24 hours a day, if necessary. They are commonly found on boiler feedwater lines where they allow the pumps to keep pumping and bypassing back to the tank when the boiler is full. This saves wear & tear on the pump and guarantees that the boiler will not be starved on water when it needs to be replenished. Another application could be to guarantee the pressure in an essential service (eg eyewash, deluge shower or even regions in the case of urban water supply applications) by ensuring that other ancillary lines are only pressurised when there is ample excess pressure above the minimum required by the priority line.
Correct sizing is essential for proper operation.
MK50 Back Pressure Regulator
US manufactured MK50 has many of the same features as the MK60 Reducing Valve but the main difference is that it regulates/controls the upstream pressure. A high quality, long lasting valve which you can safely “set & forget”. Responds rapidly to pressure changes…….better control performance than most self-acting regulators…easily maintained and simple pressure adjustment in situ. Commonly used for Pressure Sustaining, Back Pressure control or as bypass valve for boiler feedwater. When installed in-line, they are used to guarantee upstream pressure to more critical services.
It is a robust, versatile, self acting valve that has sliding gate design (rather than conventional piston or plug and seat), has hard wearing Jorcote seats with class IV shut-off and Jorlon long life diaphragm. It is suitable to use with water, oil, gas, chemicals and steam. There are many variations to the basic model such as MK51 (high sensitivity, low droop), MK501 (high flow version) and MK50HP (high pressure version). In Bronze or DI version, valves have screwed ends BSPT, BSPP or NPT, WCB or 316ss valves can be threaded, socket weld, butt-weld or flanged to Ansi 150/300 or DIN PN10/16/25/40.
Maximum inlet pressure- BZ 3,400 kpa; DI 4,100 kpa; WCB/316SS 6,600 kpa
Maximum temp- BZ (260°C)/ DI/WCB/316SS (343°C)
Pressure Control ranges:
– MK50 ranges between 14 kpa-1,034 kpa)
– MK51 (ranges between 3 kpa-1,172 kpa)
– MK50HP 1/2″-2″ (ranges between 517 kpa-3,100 kpa)
– MK50HP 2″-4″ (ranges between 172 kpa-552 kpa)
* Options: Low flow trim in all sizes, special coated seats, 316SS or Monel diaphragm, cryogenic extension, Oxygen cleaned, salt water trim, Companion flanges for Bronze & DI valves, BSPP ends
MK56 Air Loaded Back Pressure Reg
MK56 uses air (or gas) loaded on top of the diaphragm, instead of a conventional regulator where a spring applies the loading force that determines the set pressure. It has no spring or pilot and has the same rangeability (40:1) as a MK57, considered to be the most accurate regulator in the range.
By eliminating the spring, the regulator has almost no droop but the big advantage is being able to remotely control the set pressure and the wide pressure range that can be controlled. A conventional regulator might be able to regulate between 200-900 kpa but the MK56 could be loaded to provide upstream pressure control between a few kpa and then be adjusted right up to the maximum body pressure of the valve (890 psi for SS body). No spring changes..just adjust the pressure being loaded on the dome. The dome loading can be done by a loading station but the simplest method is just by using a JPR regulator from the Low Flow section or a conventional Air Filter Regulator. Adjust the regulator to a little over 5 kpa and the MK56 regulates the pressure to 5 kpa…..increase the air pressure to a little over 1200 kpa and the MK56 regulates to 1,200 kpa etc
Maximum inlet pressure- BZ 3,450 kpa; DI 4,140 kpa; WCB/316SS (6,550 kpa)
* Options: Low flow trim, stainless teel, buna or viton diaphragm, BSPP, BSPT, NPT or flanged.
Model RFT Pressure Sustaining Valve (316ss, screwed)
Elite model RFT is a 316ss self acting pressure sustaining valve suitable for fluids, air and steam.
Available in ½” → 2” (Cv’s from 2.4 to Cv25) and the main materials are 316SS with NBR or Viton diaphragm and seals.
Valve has threaded BSP female ends.
Maximum inlet pressure-2500 kpa (362 psi)
Maximum temp- 100°C or 180°C for steam
Adjutable pressure ranges – 100-500 kpa; 400-1,000 kpa
* Options: NPT, all 316SS adjusting bolt/locknut, pressure gauge