Relief Valve Blowdown Rings; D/P and Straight Runs

Readers Ask: When It Comes to Safety Relief Valves, What Is Best? Using Two Blowdown Rings or Ones? and Can a Transmitter Be Installed Above Pipe Taps?

By Bela Liptak

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Q: I would like to learn when we have to use two blowdown rings and when only a single blowdown ring in safety relief valves and which is superior?

M. Ulaganathan

A: Let me first explain the role of blowdown in regular pressure relief valves (PRV). When the PRV is closed, the system is a static one with no kinetic effects. Therefore, the pressure at the point of spring balance is equal to that in the protected tank. When the PRV is open, its inlet pressure is less than the pressure in the protected vessel because of the inlet pressure drop. This discrepancy between relieving and static conditions requires the allowance for blowdown, which is the amount of pressure by which the protected tank's pressure has to drop below the PRV's set pressure for the valve to reseat. The normal blowdown of a PRV is between 2% and 7% of set pressure. Pilot-operated PRVs can reduce the blowdown to about 2%. Industrial practice is about 7%, which means that the normal operating pressure must be under 93% of set pressure, and fired boilers require that the PRVs reach their full lift at a pressure not greater than 3% over their setpoint and reclose within the maximum blowdown values given in Table 1.

The position of the adjustable ring on the PRV nozzle controls the blowdown by establishing a secondary orifice area as the valve opens and closes. Blowdown is set by first bringing this ring all the way up to the disc (this corresponds to the maximum blowdown position) and then lowering it. If blowdown is an important consideration, field tests usually must be made after the PRV is installed. Pilot-operated valves can usually be set for smaller blowdowns. Three percent is fairly standard for a POPRV, and as little as 1% can be achieved in some cases.

In order to make fine adjustments, double blowdown rings are used (Figure 1). The upper blowdown ring is usually factory-set. The lower blowdown ring is also factory-set to achieve the appropriate code performance requirements, but it can be altered. In its top position, the valve will pop rapidly, minimizing the overpressure value, but correspondingly requiring a greater blowdown before the valve re-seats. When the lower blowdown ring is adjusted to its lower position, there is minimal restriction in the huddling chamber, and a greater overpressure will be required before the valve is fully open, but the blowdown value will be reduced.

Béla Lipták

A: I can do no better than to refer you to a comprehensive discussion of safety valves:
Introduction to Safety Valves

Richard H. Caro, CEO, CMC Associates

A: The blowdown ring on a relief valve is adjusted to set the reseat pressure of the valve. (The spring determines the pressure at which the PSV will lift, and the blowdown ring determines when it will reseat.)  A two-blowdown ring design provides the ability to adjust this reset pressure very accurately, which is required for most ASME Section 1 application (boilers). Therefore, you will typically see the two-ring design for boiler relief valves. However, most applications don't require so fine a blowdown setting, so they use the single-ring design.

Short answer, the two-ring design is better and allows tighter control of the blowdown setting, but unless you are dealing with a boiler, ASME Section 1-type valve, you usually don't need that kind of very fine adjustment.

Hunter Vegas

Q: We have a pipe which runs 30 cm above ground with an orifice plate in it, and we want to install the dp transmitter in it, which normally should be below the pipe, the elevation of which we can not change. Can we install the transmitter above the pipe taps?

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