District Energy - 2005 Fourth Quarter

Inside
Insights
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 poor condition.

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-