Set-up and design for a steam-pressure-reducing valve station
A steam pressure-reducing valve (PRV) station is an indispensable part of many steam systems. It plays a critical role by providing the correct steam pressure to process applications in a process plant. Steam enters the PRV station at a higher pressure than what is required for downstream applications, and the station reduces the pressure to the desired level before delivering steam to those applications.
Within a steam PRV station (See Figure 1), the controlling valve—which reduces the steam pressure—is typically a pneumatic control valve. However, a regulating valve (also called a PRV) may sometimes be used instead. In either case, you’ll want the controlling device to have a long operational life so you can reduce maintenance needs, downtime, and total cost of ownership for the PRV station.
Thanks to current available technology and better materials, PRV station components should last at least six years. But you can likely do better than that. Through proper system design, component selection, and installation of your steam PRV station, you may be able to greatly prolong the life of all the steam components. Let’s review several best practices that can help you achieve this goal.
1. Match the valve to the application
To select the correct PRV for a given application, you need to know the maximum and minimum steam flow rates, as well as understand the turndown capabilities of the pneumatic control valve and/or regulating valve.
Although maximum steam flow rates are seldom, or never, reached in the operation of the steam PRV station, be certain the PRV meets or exceeds this rate. Just as important, the PRV needs to accommodate the minimum flow rate, as it will be a frequent and important control point. To summarize: the steam PRV must be able to operate successfully at the minimum and maximum steam flow rates.
Knowing the turndown capabilities of the pneumatic control valve and/or regulating valve will help you make an appropriate selection. As a general rule of thumb, typical turndown ratios for select valve types include:
- Regulator: 20 to 1
- Globe valve: 30 to 1
- Cage valve: 40 to 1
In addition to meeting the requirements for a given application, all valves in a steam PRV station—including control and isolation valves—also need to meet permissible internal leak rate standards as designated by FCI/ANSI or API. The standards denote the tolerance level for steam leaking across the valve seat to pass downstream or to the atmosphere. FCI/ANSI designates six permissible leak rates, or classes, numbered I through VI. The higher the leak rate number, the lower the permissible internal leak rate.
Therefore, a Class I valve will have the highest internal leak rate and usually the lowest cost, while a Class VI valve will have the lowest rate and typically the highest cost. In a steam PRV station, all the isolation and PRV valves should be Class IV or higher.
2. Specify a noise level of 85 dBA or lower
Setting an upper noise limit of 85 dBA for a PRV is more a function of managing outlet velocity and extending valve life, with the added benefit of reducing noise. PRVs that have high dBA levels will also have high outlet velocities and reduced operational life. A PRV with low dBA levels, or lower velocities, will have a much longer reliable operational life.
There are many ways to lower the dBA level in a PRV application, including increasing the valve outlet pipe size, muffling the orifice, or adding special trim. The valve manufacturer will provide the appropriate pipe size required after the PRV to achieve the desired dBA level. One can also use a muffling orifice plate (See Figure 2) to reduce a high-pressure drop across the steam valve to reduce velocities. In addition, special trim can be used to minimize velocity and noise.
3. Include a steam line drip pocket
All steam PRVs must have a condensate removal drip pocket piped upstream of the valve. The drip pocket removes condensate from the steam line, preventing condensate from passing through the valve. This is essential because condensate that enters the steam valve will cause erosion and shorten the life of the valve. Even during those times when the steam PRV shuts off during low or nonproduction periods, the condensate drip pocket will continue to remove accumulating condensate from the inlet of the valve.
4. Include a strainer with a blow-off valve
A strainer is also a requirement upstream of a steam PRV to protect the valve from materials that may corrode the system. Steam lines frequently contain residual solid materials from corrosion in the steam line. The strainer will filter the steam stream and prevent this material from lodging within the valve, which could otherwise cause premature failure. The strainer should be rated for 20 perforated stainless steel mesh.
Never mount the strainer with the strainer segment in a down position; instead, install the strainer segment in a horizontal position. This will prevent condensate from accumulating in the strainer pocket and eventually passing through the PRV, thereby reducing the likelihood of internal erosion and premature valve failure. Finally, install a blow-off valve on the strainer to allow plant personnel to periodically clean out the strainer.
5. Locate the pressure reducing valve
Placing the steam PRV correctly in the station helps to ensure proper system operation. Make sure the distance after the steam PRV is at least 10 pipe diameters before any change in steam flow direction or before the steam line takes offline. The regulating valve should be at least 20 pipe diameters ahead of a direction change as well.
6. Position the pressure-reducing valve
To extend the life of PRVs, always install them on horizontal steam lines, never vertically. A PRV in a vertical installation has no ability to eliminate the build-up of condensate prior to the valve’s inlet. Condensate passing through a steam PRV always negatively affects the valve’s life.
7. Use bypass valves and warm-up valves
Bypass valves and warm-up valves should be used in all PRV installations. The warm-up valve warms the steam line within recommended time frames for the steam line. The warm-up is modulated and controlled during the start-up procedure. A steam PRV should not be used for warming up a steam distribution line.
The bypass valve must have a lower flow coefficient (Cv) than the PRV. When using the same diameter bypass valve as the PRV, the safety valve will have to be sized for the bypass valve, which always will have a higher Cv. Sizing the safety valve for the bypass valve will usually require the safety valve to be extremely large in size and capacity.
8. Check if you need a safety valve
Safety valves are an important consideration in a steam PRV station. But they may not be required in every case. If any steam component or the steam line downstream of the steam PRV is not rated for the maximum inlet steam pressure to the PRV station, then a safety valve must be installed to protect the system.
The safety valve needs to be sized for the maximum steam flow with the highest steam pressure that could be provided to the PRV. In addition, to ensure the proper safety valve size, perform calculations with the largest Cv trim available for the PRV. When installing a safety valve, be sure the discharge is piped to a location where it will not pose a safety risk for plant personnel.
9. Determine when more than one valve is required
There will be times when a steam PRV station requires more than one PRV. For example, when the steam flow varies greatly and one valve does not have the required turndown capabilities, an additional valve—or valves—will be required to achieve the desired outlet pressure. In systems with more than one PRV, the safety valve must be sized based on all valves failed in the open position and have the largest trim available from the manufacturer.
Note that the piping from the PRV to the downstream isolation valve should be designed and installed to meet the highest steam pressure at the inlet to the PRV. The downstream isolation valve is located before the safety valve that protects the system. Therefore, if the isolation valve is closed and the PRV opens, the piping could experience the inlet steam pressure.
10. Install before and after pressure gauges
It is always good practice to install pressure gauges before and after the steam PRV as diagnostic tools. Be sure to include a siphon pipe and isolation valve for maintenance purposes.
11. Establish a Standard Operating Procedure
Every PRV station requires a SOP to ensure that plant personnel are starting, operating, and shutting down the valve station correctly and safety. Valve manufacturers should provide a SOP as part of the documentation with the equipment.
By paying careful attention to proper system design, component selection, and installation practices, you can prolong the life of components in your steam PRV station. Be sure to match the PRV to the application, minimize velocities, include drip pockets and strainers, and position components properly. Following the guidelines above, you can reduce maintenance headaches and total cost of ownership for your PRV station.
Kelly Paffel is technical manager for Swagelok Energy Advisors, Inc.