Steam Pressure Reducing Valve

What is a steam pressure reducing valve? 

As such, steam reducing pressure valve is defined as an automatic control valve used to reduce unregulated higher inlet pressure to a reduced and constant outlet pressure irrespective of fluctuations in upstream steam pressure.  A steam pressure-reducing valve is a valve used in many steam applications to control flow of steam. This valve helps in critical roles to provide the correct level of steam pressure in plants. Steam gets in a steam pressure reducing valve at a high pressure relative to what is needed downstream. The valve helps to reduce the pressure to the required level before it is delivered to the needed location. Steam pressure reducing valves are used in industrial, commercial, and institutional applications to control steam pressure. It is used in areas such as testing fixtures, oil lines, autoclaves, single radiators, steam irons, and vulcanizers. A steam pressure reducing valve is an automatic valve used to ensure there is no excess pressure in a system. This valve is also known as a steam pressure reducing regulator.

Figure: Steam pressure reducing valve

How does a steam pressure reducing valve work? 

In its normal position before start-up, the main valve is closed and the pilot valve opens by spring air pressure or spring force. Steam passes via the pilot valve to the diaphragm chamber and out via the control orifice. When the flow through the pilot valve becomes more than the flow via the orifice, the diaphragm control pressure increases and the main valve opens. When the steam flows via the main valve, the downstream pressure increases and it feeds back through the pressure sensing line to the lower side of the pressure diaphragm. As the compression force and the force below the diaphragm balance, the pilot valve throttles. The main diaphragm control pressure maintained places the main valve to release enough steam needed for delivery pressure. Adjusting the air pressure or spring above the diaphragm alters the downstream pressure. When no steam is required, the sensing line pressure is increased which closes the pressure pilot while control pressure flows back via the control orifice. This helps the main valve in holding the needed pressure and it can close tight to enhance dead-end shutoff. 

Figure: Working of a steam pressure reducing valve

Types of steam pressure reducing valves

Direct-acting steam pressure reducing valve

These are steam pressure reducing valves suitable for use in small loads where no specific pressure control is needed. Steam pressure reducing valve manufacturers design these valves in a way to make them have a compact size, easier installation, and cheap. However, these valves set pressure is of more variations relative to the pilot-operated steam pressure reducing valve. These valves are used in a point-of-use installation.

Direct-acting steam pressure reducing valve is a simple valve that operates using convoluted bellows or flat diaphragm. The operation of this valve needs no external sensing line downstream since it is self-contained. Direct-acting steam pressure reducing valves have adjustment springs that move to help in opening the valve directly. Compression of the spring enhances opening force on the steam pressure reducing valve to increase flow. When pressure increases downstream, equilibrium occurs where downstream pressure feeds to a lower side of the adjusting spring causing upward force to counterbalance spring compression. The compressive force of the spring is restricted to enhance enough sensitivity to equalize changes in downstream pressure. This results in simple pressure control via valve orifice. 

Figure: Direct-acting steam pressure reducing valve

Pilot operated steam pressure reducing valve

These are steam pressure reducing valves that are used for large loads that need close pressure control. These valves offer a fast response to variation in load and are suitable for a broad range of steam flow rates relative to the direct-acting valves. The pilot-operated steam pressure reducing valves are made of larger size and are more costly.

This valve employs a pilot valve to load the diaphragm or piston to increase the downward force needed to open the main larger valve. This approach helps to enhance larger flow capability with lower variation in pressure. When the force between secondary pressure and adjusting spring is balanced, the closing and opening of the pilot valve are controlled. This type of pilot-operated steam pressure reducing valve helps to deliver pressure to the primary valve diaphragm or piston. The area of the diaphragm or piston helps to amplify the downward force produced by the pilot flow pressure. This helps to open a larger main valve to provide the capacity for high flow rates. 

Due to the force amplification, opening the pilot valve by small change leads to large flow change and downstream pressure via the main valve. As such, a small change in adjusting spring force results in rapid response over a broad range of flow rates. One of the advantages of the pilot-operated steam pressure reducing valve is the quick response and tight control of downstream pressure. Pilot-operated steam pressure reducing valves are of two types that are internally piloted piston and externally piloted valve. The internally operated piloted piston valve employs two valves main valve and the pilot valve in one unit. This valve can provide +/- 5% accuracy. 

The externally piloted valve employs a double diaphragm which replaces the piston operator in the internally operated valve. This externally piloted steam reducing pressure valve can provide +/-1% of accuracy. 

Figure: Pilot-operated steam pressure reducing valve

Practices that help to enhance the life of a steam pressure reducing valve station 

Ensure the steam pressure reducing valve matches the application

Select the correct valve for that specific application. This means first to find out the minimum and maximum steam flow rates. The steam pressure reducing valve needs to be able to handle the minimum flow rate since it will be the most common and critical point to control. 

Noise level be 85dBA or lower

Setting the highest noise level to be 85dBA for the steam pressure-reducing valve will help manage velocity at the outlet and extend valve life as well as help reduce noise. Steam pressure reducing valves that have outlet velocity very high have short operational life. If the steam pressure reducing valve has a low noise level or works at very low velocities, it will have a very long service life. Some of the ways to reduce noise levels in these valves are to increase the size of the outlet valve pipe or to add a special trim. 

Add steam strainer that has a blow-off valve

This strainer is required upstream of the steam pressure reducing valve to protect it from corrosive materials. Steam lines tend to have residual materials from corrosion. Using strainer helps to filter stream and thus prevent such materials from lodging in the valve which could cause premature failure. 

Add steam line drip pocket 

A steam pressure-reducing valve will need a drip pocket for the removal of condensate piped upstream. This helps to remove condensate from steam lines and thus prevent condensate from passing via the valve. The importance of this feature is that it will ensure condensate cannot enter the valve because it can cause erosion and short service life. 

Location of the steam pressure reducing valve 

The proper location of the valve in the steam station helps to enhance proper working. As such, ensure that after the steam pressure reducing valve the minimum distance is 10 pipe diameters before a steam line or before changing direction of steam flow. 

Orientation of the steam pressure reducing valve

Steam pressure reducing valves will have more service life if installed in horizontal orientation than in vertical position. When installed in a vertical position, the valve will not have the ability to remove condensate build-up before the valve’s inlet. This is because as the condensate passes through a steam pressure reducing valve it affects the life of the valve negatively. 

Employ warm-up and bypass valves

In a steam pressure reducing valve station, a warm-up valve will help to warm the steam line in the range of the recommended time frame for that steam line. The by-pass valve will need to have a flow coefficient lower than that of the steam pressure reducing valve. 

Installation of pressure gauges 

Steam pressure-reducing valve station will need pressure gauges before and after. This will help to diagnose the valve. Also, it would be necessary to install an isolation valve and siphon pipe to help when doing maintenance. 

Applications of steam pressure reducing valves

• Used to reduce varying or steady inlet pressure to enhance constant adjustable outlet pressure. In this case, the incoming pressure operates the valve. As the outlet pressure comes close to the spring setting on a pilot, the steam pressure reducing valve starts modulating and maintains set pressure. 
• Low outlet pressure for the delivery of steam. 
• Provide make-up steam for the turbine exhaust main. When the load in a turbine reduces and the exhaust of the turbine is not enough for steam load, some slight drop in exhaust pressure makes the valve feed the right quantity of steam as per the demand. 
• Distribute steam from overloaded boilers in essential services. 
• Sterilizers and unit heaters to lower varying or steady inlet pressure to constant delivery pressure. 

Advantages of steam pressure reducing valves 

• Do not need an external power source. 
• Employ a simple design that makes these valves low in cost. 
• They offer a fast response. 
• These valves do not need feedback controllers or separate measuring elements. 
• Highly reliable and easy to maintain. 
• These valves eliminate external leakage and high friction as they do not have stem packing. 

Disadvantages of steam pressure reducing valves 

• Steam pressure-reducing valves have no reset action. 
• These valves have limited pressure ratings and sizes. 

Troubleshooting steam pressure reducing valves 

Under normal load the controlled pressure over-shoots

• There is foreign materials or dirt lodged between the pilot valve head and seat. Loosen the screw and remove tubing connections to help in cleaning or replace if necessary. 
• Foreign materials between the main valve seat and head. Do inspection and clean the seat and head. 
• The pressure sensing line or orifice is plugged. Remove, do an inspection and clean the lines and orifices. 

Steam pressure reducing valve controlled pressure overshooting only under light loads

• The seat and head of the main valve are worn or they have dirt between them. Do inspection and clean the seat and head. Over-size valve. Adjust the screw to provide the needed pressure for light loads. 
• A bypass valve is leaking or it is not shut tightly. Check the bypass valve and repair it as necessary. 
• Foreign material or dirt lodged on the main valve. Do inspection and clean that dirt. 

Steam pressure reducing valve is not opening 

• Rupture on the main valve. Unscrew tubing connections and crack bypass valve. In case there is a flow of steam from the main valve, the diaphragm is defective and needs to be replaced. 
• Plugged orifice. Remove the orifice and clean. 
• Dirt plugged in the pilot valve seat. Remove the seat and head assembly. Inspect for dirt and clean or replace as necessary. 
• Blockage in the pipeline strainer. Inspect and clean the strainer. 

Steam pressure reducing valve has low delivery pressure 

• Adjustment of the pilot valve is not properly done. Adjust the screw to the correct pressure. 
• Undersized valve. Check the required load against the rating on a valve to confirm if it is the main cause. 
• Very low steam supply pressure. Check the supply pressure and correct as necessary. 
• Ruptured main diaphragm valve. Unscrew tubing connection and crack bypass valve. In case there is a flow of steam from the diaphragm, then the diaphragm needs to be replaced since it is defective. 
• Missing bleed orifice. Replace proper fitting. 

The steam pressure reducing valve is not closing 

• Leakage on bypass valve or this valve is open. Check and close or repair as necessary. 
• The pilot sensing line is not installed or is blocked. Remove, inspect, install, or clean as necessary. 
• A pilot is a rupture thus steam. The pilot diaphragm assembly needs to be replaced. 

Summary 

Steam pressure reducing valves are valves that are used to enhance precise downstream control of steam. These valves can automatically adjust the opening of the valve to help make the pressure remain constant even when there is fluctuation in flow rates by springs, diaphragms, or pistons. Steam pressure reducing valves employ closing and opening components in the valve body to alter the steam flow, reduce steam pressure, and control level of closing and opening degree of the parts using pressure behind the valve.

There are two types of steam pressure reducing valves that is direct-acting steam pressure reducing valve and pilot-operated steam pressure reducing valve. The direct-acting steam pressure reducing valve manufacturer makes the valve for use where there is a small load and where downstream pressure drop can be accepted. These direct-acting valves are best suited for light-load applications. The pilot-operated steam pressure reducing valve manufacturers make the valves in a way that makes them respond easily to changing loads while keeping stable secondary pressure with precise control of pressure.

The pilot-operated steam pressure-reducing valves are best suited for large load applications. Steam pressure reducing valves are used in many industries such as commercial, institutional, and industrial applications among others. Uses of these valves include single radiators, vulcanizers, autoclaves, testing fixtures, oil lines, steam irons among others to control steam flow. The steam pressure reducing valves have several advantages that make them widely used in different industries such as simple design, low cost, no external power needed, highly reliable and easier maintenance, the fast response among others.