Valve Magazine - Winter 2012

c) Modify the outlet piping by making it larger or shortening the length of pipe, thereby reducing the effects of built-up backpressure.

OVERSIZED VALVE

While most PRVs are protecting equipment for more than one relief event, the size of the valve is based on the scenario requiring the greatest relieving capacity. An example would be when a PRV is sized for both fire and blocked outlet scenarios. The fire sizing requires significantly greater orifice area than the blocked outlet sizing. However, since the blocked outlet scenario is more common and more likely to occur, then the PRV will be potentially starved for capacity, causing the valve to “chatter” (rapidly opening and closing). Valve chatter, as well as flow instability, could inevitably cause valve damage such as premature fatigue failure of the bellows, as well as galling of guiding surfaces. In our experience, a PRV should not be specified that has an actual orifice area more than 3 to 5 times larger than the required area.

Mitigation strategies for failure in
this scenario include:
a) Install multiple PRVs and stagger
the set pressure for each of the
scenarios. Ensure the small valve
is properly sized based on the low-
est required capacity relief sce-
nario.

HIGH INLET
PRESSURE DROP

When the length of pipe leading to the inlet of the PRV causes an excessive pressure drop to a level below its reseat pressure, the valve will begin to oscillate open and closed, potentially leading to chatter. This situation can lead to bellows failure caused by premature fatigue. A bellows with this type of failure will typically have a horizontal crack along its convolutions or a complete fracture.

Mitigation strategies in this case
include:
a) Use a pilot valve and remote
sense the pressure-pickup point,
locating the pickup directly on or
closer to the equipment under
protection. This will ensure the
valve function is based on the
pressure at the equipment and not
the inlet of the PRV.
b) Install a modulating pilot-operat-
ed valve that will open in propor-
tion to the upstream pressure and
be less susceptible to chatter from
inlet line losses.
c) Lower the set pressure of the
PRV, which will reduce the capac-
ity of the PRV. Keep in mind the
operating pressure and the typical
blow-down may not allow this.
d) Modify the inlet pipe to reduce
inlet line pressure losses.
e) Raise the blow-down ring, which
lengthens the blow-down time.
While it is difficult to quantify the
increase in blow-down, in general,
raising the ring will reduce chatter.

OPERATING TOO CLOSE
TO SET PRESSURE

Operating a system too close to the set pressure of a PRV can cause a valve to be in a state of simmer. When the valve is between simmer and fully open, this can be classified as “flutter.” Although this state is neither open nor closed, the disc of the valve will have a very small oscillation and may cause galling in the guiding surfaces, as well as fatigue failure of the bellows. The American Petroleum Institute recommends a 90% operating-to-set pressure ratio.

Recommendations for mitigating
this type of failure include:
a) Reduce the operating pressure.

For the cases described in this article, we assumed that all calculations have been reviewed by a qualified engineer. Prior to implementing any of these recommendations, please consult an applications engineering team to verify acceptable mitigation strategies. VM

SEAN CROXFORD is business operations manager, Farris Engineering Services. Reach him at scroxford@curtisswright.com. Martin Joyce is applications engineering manager, Farris Engineering. Reach him at mjoyce@curtisswright.com.