When Is the Last
Time You Had a Date
With Your PRV?
Steve Tredinnick, P.E., Mechanical Systems, Affiliated Engineers Inc.
Editor’s Note: “Inside Insights” is a column
designed to address ongoing issue of interest to
building owners, managers and operating engineers who use district energy services.
Even though we are well into the
heating season now, it is never too
late to get to know your pressure-reducing valves (PRVs) more intimately. I
am not talking a candlelight dinner and a
bottle of wine – just some quality time
with one of your significant heating components. For steam district heating customers
who are connected to a steam system, periodic inspection and proper cleaning of PRVs
will fuel a long and satisfying relationship.
As stated in last quarter’s column,
PRVs take steam from a higher pressure
and reduce it to a lower pressure. There
are two basic types: direct-operated or
pilot-operated. Pilot-operated valves are
typically used in district heating applications since they have better accuracy and
rangeability than direct-operating valves
and are better suited for larger capacities
with fluctuating supply pressures. Considered
self-contained, pilot-operated valves are
comprised of a main reducing valve and an
integral pilot valve that uses upstream steam
as a signal force to the main valve diaphragm
and varies the signal pressure in proportion
to the downstream pressure plus an
adjustable spring force.
The ‘signal’ is transmitted via a pilot
line that extends from the pilot valve to the
low-pressure side of the reducing station.
The pilot valve sensing control line as well
as the regulator external control tubing is
only 1/4-inch diameter stainless steel
(fig. 1). The external control tubing conducts steam from the pilot to the main
valve diaphragm and bleedport. Sediment
can build up in the pilot valve external
control tubing due to its small diameter,
especially at the restriction elbow at the
bottom of the main valve and at the
bleedport (locations 4a and 5a, respectively). Therefore, cleaning at these locations
should be performed every six months
depending on the steam quality and the
condition of the piping.
Other PRV parts are subject to normal
wear and must be inspected periodically
and replaced as necessary. The frequency
of inspection and replacement depends on
the severity of service conditions and on
applicable codes, government regulations
and customer maintenance standards.
Accordingly, systems that operate year-round with a heating load should be
looked at more frequently. A valve kept
relatively free of dirt will function for years
with minimum attention. Under normal
circumstances, complete dismantling of
the valve at regular intervals is not recommended unless the valve is in extremely
Other recommendations to maintenance or operation:
Install the reducing valve so it is easy to
remove and maintain.
Install a strainer upstream of the reducing valve to intercept and protect it from
any debris dislodged in the piping.
Provide manufacturer’s recommended
straight diameters of pipe before and
after the reducing valve.
If silencers or other devices are installed
immediately after the reducing valve,
provide a means to drain the trapped
vapor and condensate to assist in removing the reducing valve. On the upsteam
side, this is usually achieved with a blow-
Figure 1. Typical Pilot-Operated Steam Valve.
No. 4A BLEEDPORT
No. 8B TEE
No. 5A RESTRICTION ELBOW - TYPE E
Courtesy Spence Engineering Co. Inc.