Pressure reducing valves (PRV) are valves designed to lower the incoming pressure of steam or gas to a desired outlet pressure. This allows for safer and more efficient operations, particularly in boiler systems that require lower pressure for specific applications.
Why Reduce Steam Pressure?
Steam systems often operate at high pressures. There are several reasons to reduce steam pressure, including:
1. Improved Energy Efficiency: Steam at lower pressure condenses at a lower temperature, allowing more heat energy to be absorbed by the process.
2. Application-Specific Requirements: For instance, heating water to 120°F with low-pressure steam at 220°F is more efficient and easier to control than using high-pressure steam at 350°F. Lower pressure steam also reduces the potential for overheating and limits leakage across control valves.
Important note: Many processes step high pressure steam down to low pressure at the point of use to save BTU’s. However, the heat exchangers become bigger due to the superheat, higher volume of the low pressure steam, and lower temperature difference between the steam and the fluid being heated. When factoring for the PRV and larger heat exchanger needed, this is the more expensive option, but it is typically more efficient.
Direct Operated PRV
Types of Pressure Reducing Valves
Direct Operated PRV
Direct operated pressure reducing valves are typically used in lower-capacity applications. These PRVs function using a spring and a diaphragm, often made of stainless steel. You can make adjustments to a nut on the top of valve that allows you to adjust the outlet pressure.
- Advantages: Simple design, inexpensive, fewer parts, easier installation (no pilot line required).
- Disadvantages: Lower capacity and less accurate compared to pilot-operated PRVs.
Pilot Operated PRV
Pilot Operated PRV
Pilot operated PRVs are more common in industrial plants, handling higher capacities and providing greater accuracy. These valves essentially consist of two valves in one—a pilot valve and a main valve. The pilot valve controls the flow to the main valve via a sensing line that is reading pressure downstream. The idea behind using a sensing line is that the pressure profile will be more stabilized downstream than directly after the valve.
- Advantages: More accurate, higher capacity, lower droop.
- Disadvantages: More parts, potentially more maintenance, and requires a sensing line.
PRV Piping Considerations
When installing a pressure reducing valve , proper piping practices are critical to ensuring smooth operation. Many times when people have problems with pressure reducing valves it is because they have some sort of piping issue. Below are some piping considerations:
Photo courtesy of Armstrong
- Drip Leg and Trap: Install a drip leg and drip trap upstream of the PRV to prevent water hammer and condensate issues.
- Strainers: Always install strainers ahead of PRVs, particularly in systems using pilot operated valves because they have very small clearances. The smallest amount of debris can hang up the pin and cause the valve to blow through.
- Bypass Line: Installing a bypass line with a globe valve allows for system maintenance without shutting down the entire steam system.
- Sensing Line: For pilot-operated PRVs, ensure the sensing line is straight (not flex tube) and slopes towards the downstream side to prevent condensate buildup. Condensate will make it so the PRV can’t sense the downstream pressure. . A good rule of thumb on the pitch is 10 to 1.
- Pressure Gauge: Every PRV requires a downstream pressure gauge for adjustment during system operation. Every PRV needs to be adjusted after installation, while there is a load on the system.
Troubleshooting Pressure Reducing Valves
Low Downstream Pressure
If the downstream pressure is lower than expected, the diaphragm is usually the first place to check. A broken diaphragm is a common issue. If the diaphragm is intact, verify that the valve has sufficient capacity for the application.
High Downstream Pressure
High downstream pressure typically indicates that the valve or seat is leaking. This is often due to debris in the valve or worn valve seats that need to be re-lapped. For pilot-operated PRVs, follow manufacturer procedures to diagnose whether the leak is in the pilot valve or the main valve.
Limitations of Pressure Reducing Valves
While pressure reducing valves are effective, they do have limitations:
- Maximum Pressure Reduction: The highest ratio of pressure reduction from inlet to outlet is 10:1. For instance, you cannot reduce pressure from 150 psi to 2 psi. Instead you can only go down to 15 psi.
- Minimum Pressure Drop: A typical PRV requires a minimum pressure drop of 20% to function properly. For example, if your boiler load fluctuates between between 100 and 125 psi, a PRV will not allow you to maintain 100 psi downstream. When pressures were at 100 you would be at 80 psi or 20% pressure drop. In this application a control valve would be needed.
- Dead-End Service: PRVs are not designed for dead-end service. In situations where there is no load downstream, PRVs have an allowable leakage that will eventually cause safety relief valves to pop.
Outlet Pressure Range And Sizing
PRVs come with a specific spring that determines their outlet pressure range. Often, these springs are color-coded to indicate their range, but markings may not be visible on the valve itself. When selecting a PRV, ensure you choose one with the proper spring range for your desired outlet pressure.
Do NOT size a PRV based on line size alone. PRVs are often at least one pipe size smaller than the line they are installed in. Oversizing a PRV can lead to control issues, excessive leakage, and the need for larger safety relief valves. (This is because your safety relief valve is sized based on being able to handle all of your PRV capacity.) Properly sizing a pressure reducing valve is essential for optimal performance. Here are the key factors:
- Inlet Pressure
- Desired Outlet Pressure
- Flow Rate (measured in lbs/hr)
Conclusion
Pressure reducing valves are an integral component in steam and boiler systems, providing precise control over steam pressure to ensure safe and efficient operation. Its important to understand the different types of PRVs, how to size them, and how to troubleshoot common issues. Whether you’re dealing with small applications or large industrial loads, choosing the right PRV for the job is crucial.
