Automatic Control Valves, Pressure Reducing Valves

A Pressure Reducing Valve (PRV) is an Automatic Control Valve designed to reduce a higher unregulated inlet pressure to a constant, reduced downstream (outlet) pressure regardless of variations in demand and/or upstream (inlet) water pressure.
Pilot Operated Pressure Reducing Valves

The throttled position of the main valve is controlled by an adjustable pilot valve operating in conjunction with an orifice (or needle valve). The pilot valve senses the downstream (outlet) pressure and reacts immediately as the outlet pressure increases or decreases with varying demand.

The pilot senses the downstream (outlet) pressure and reacts immediately to add or remove water from the top of the main valve diaphragm assembly, allowing the valve to reposition and throttle as the outlet pressure tends to increase or decrease with varying flow demand. The pilot diaphragm will automatically sense the changes in the flow of the system as it continuously controls the main valve to throttle or to open or close and maintain the desired, preset reduced outlet pressure. The throttling action of the main valve provides the required reduced downstream pressure.

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unctions of Pressure Reducing Valves

A pressure-reducing valve is able to control pressure through the fully automatic self-contained operation without the necessity of an external power source. The main functions of a pressure-reducing valve are:

In steam systems, pressure-reducing valves are used to get precise control of downstream pressure. These valves automatically adjust the valve opening and adjust the pressure to keep it constant during pressure fluctuations.
Properly selected pressure-reducing valves can be used for water hammer protection under defined conditions.
They can also be used as bypass valves for saving the system during power failures.
Pressure-reducing valves are capable of taking rapid action by immediate sensing and adjusting based on the downstream pressure.

Types of Pressure Reducing Valves

A pressure-reducing valve in a steam system works by balancing the steam pressure with a spring. Most of the modern pressure-reducing valves are manufactured using this basic concept. Based on the mechanism of controlling the valve opening, pressure-reducing valves are classified into two types:

Direct acting pressure reducing valve and
Pilot-operated pressure-reducing valve.

Direct-acting pressure-reducing valve:

Direct-acting pressure-reducing valves are ideal for small loads where precise pressure control is not required. They are manufactured in a compact size, cheap, and very easy to install. However, they usually have more variation from the set pressure as compared to their pilot-operated counterpart.

Direct-acting pressure-reducing valves are designed for point-of-use installation. This is the simplest type of pressure-reducing valve that operates with either a flat diaphragm or convoluted bellows. It does not need an external sensing line downstream to operate as it is self-contained. The accuracy of direct-acting PRVs is typically (+/-) 10% of the downstream set point.

In direct-acting pressure-reducing valves, the movement of the adjustment spring aids in the valve opening directly. The spring compression creates an opening force on the pressure-reducing valve which increases the flow. As pressure builds downstream, equalizing occurs by feeding the downstream pressure to the underside of the adjustment spring where its upward force counter-balances against the spring compression. The spring compressive force is limited to allow sufficient spring sensitivity to equalize with downstream pressure changes. This causes a simple pressure control through a valve orifice. However, high flow rates can cause pressure variation.
Pilot Operated pressure reducing valve:

Pilot-operated pressure-reducing valves are normally used for larger loads requiring close pressure control. They provide a faster response to load variation and are suitable for a wider range of flow rates as compared to the direct-acting types. They are larger in size and costly.

A pilot-operated pressure-reducing valve uses a pilot valve to load a piston or diaphragm that increases the downward force used to open a larger main valve. This helps in larger flow capacity with a lower pressure variation. By balancing the force between the adjustment spring and the secondary pressure, the opening and closing of the pilot valve are controlled. This pilot valve purposely delivers pressure to the main valve piston or diaphragm. The downward force created by this pilot flow pressure is amplified by the area of the piston or diaphragm that enables the opening of a much larger main valve providing the ability for very high flow rates.

Due to this amplification, a small change in the opening on the pilot valve results in a large change in flow and downstream pressure through the main valve. Therefore, a little change in adjustment spring force on the pilot accomplishes a quick response over a wide range of steam flow rates. The main advantage of a pilot-operated pressure-reducing valve is the quick response and tight downstream pressure control.

There are two types of pilot operated pressure reducing valves:

Internally piloted piston operated that incorporates two valves-a pilot and main valve-in one unit. It provides an accuracy of +/- 5%.
Externally piloted where double diaphragms replace the piston operator of the internally piloted design. this type of pressure-reducing valve provides an accuracy of +/- 1%.

The image in Fig. 1 shows the components of pilot operated pressure-reducing valve
pplications of Pressure-Reducing Valves

Self-acting pressure-reducing valves are used in the following services:

Air or gas services: Both direct-acting and pilot-operated pressure-reducing valves for air and gas services are used in compressed air systems, power tools, pneumatic control systems, and control valves for industrial gas storage and distribution systems. The selection of the type of pressure-reducing valve for these services is based on the accuracy of control required.

Water services: Pressure-reducing valves find wide application in domestic and industrial water distribution services and fire protection systems. In general, direct-acting pressure-reducing valves are preferred for these services. During high-demand scenarios, maintaining system pressures becomes very difficult. Pressure-reducing valves are used in those lines which can effectively control the pressure downstream to an acceptable limit.

Steam services: The majority of the pressure-reducing valves are used in steam applications involving direct steam supply, steam engines, and turbines.

Other Services: Pressure-reducing valves find their applications in the following:

Bearing lubrication systems in heavy industrial equipment and rolling mills.
In hydraulic presses to control ram pressure.
To control the pressure in fuel-oil systems.
To reduce pressure in sterilizers, humidifiers, unit heaters, and small process equipment

Advantages of Pressure Reducing Valves

The main advantages of a pressure-reducing valve are:

No requirement for an external power source.
Separate measuring elements or feedback controllers are not required.
Simple design with low cost.
High reliability and easy maintainability
External leakage and source of high friction are eliminated by the absence of stem packing.
Fast response

The pressure reducing valve may be used for water hammer protection under properly defined conditions. The water hammer events are so fast that the relief of pressure by this means requires very special rapid response valves designed for the particular system.

Pressure relief valves may also be used as an added precaution where some other method is in place as the basic protection device, such as an air vessel.

There are also by-pass valves which are required to operate when a pump stops due to a power failure or normal trip, anticipating the eventual return flow, and then being required to be closed to prevent the excessive waste of the return flow.

Figure 17.1 shows the basic system configuration for the installation of the PRV, where the PRV at (1) is a subsidiary type, maybe at the pump flange and the PRV type at (2) is the anticipating valve.
Spring-Loaded Pressure-Reducing Valves

The spring-loaded pressure-reducing valve (Figure 7-15) is commonly used in pneumatic systems. It is often referred to as a pressure regulator. The valve simply uses spring pressure against a diaphragm to open the valve. On the bottom of the diaphragm, the outlet pressure of the valve forces the diaphragm upward to shut the valve. When the outlet pressure drops below the set point of the valve, the spring pressure overcomes the outlet pressure and forces the valve stem downward, opening the valve. As the outlet pressure increases, approaching the desired pressure, the pressure under the diaphragm begins to overcome spring pressure, forcing the valve stem upward and closing the valve. You can adjust the downstream pressure by turning the adjusting screw, which varies the spring pressure against the diaphragm. This particular spring-loaded valve will fail in the open position if a diaphragm rupture occurs.
Control Loop Interlock

There may be specific interlocks for the loops. If the temperature at the outlet of the desuperheater (after PRV; i.e., at auxiliary steam header) is greater than the set value. then the PRV (both high- and low-capacity) will be closed. If there is a signal failure, the associated solenoid valve will be closed from the DCS to secure fail lock condition of the associated valve (Figs.11.18 and 11.19). If the temperature is greater than the set value sensed by LVM, the associated PRV will be closed.
Pressure control

During start-up and shutdown, when live steam is used, the pressure at the glands is controlled by the pressure reducing valve in the live steam supply line. As load increases, the steam flow to the HP/IP glands progressively reduces and eventually reverses, as leak-off steam becomes available. Consequently, the rise in pressure at the glands causes the live steam pressure regulating valve to close progressively, thus maintaining a constant supply pressure at the glands and eventually closing completely. The LP glands are now sealed by steam leaking from the HP and IP glands, and the pressure at the glands is controlled by a leak-off valve which dumps steam to an LP heater.

This arrangement ensures that only one regulating valve is in control at any one time and that the changeover from live steam to leak-off steam is fully automatic.

The steam pressure in the sealing line is indicated in the control room and locally; a fault condition is indicated by low pressure alarms.
Localization for fresh water supply

Numerical methods make it possible to predict the effect of replacing one or more pressure reducing valves (PRVs) with PATs in a network [40]. The sustainable management of WSSs can be achieved by monitoring and controlling losses, which requires a deep knowledge of the network, in particular its characteristics and operating mode [33]. In recent years, significant research effort has been committed to proposing a general numerical model for the optimal location of the energy recovery node using different criteria, including leakage reduction and power plant production [16,32,48]. A complete analysis of these contributions is out of the scope of this paper, but a comprehensive review can be found in Fecarotta and McNabola [24]. At the present state of knowledge, these general models can handle only small hydraulic networks. As a result, in most of the real-life situations more simplified methods for PAT localization and sizing are implemented.

Nevertheless, the problem of the optimal location of PATs within a water distribution network is still an unresolved problem. Thus, the
Compressible Flow Relationships

The most commonly used device for regulating gas flow in pipelines is the pressure reducing valve. These valves provide a mechanical constriction in the pipe, much as an orifice plate reduces the cross-sectional flow area, thereby effecting a local pressure drop in the flow. Commercial reducer valves are much more sophisticated than orifice plates, but they work on basically the same principle. The inner workings and geometry of the valve seating and construction, called the valve trim, are today often designed for quieter operation than was the rule with the older, more conventional globe and gate valves. Fig. 4.20 is an illustration of some commercially available control valves, and Fig. 4.21 is a schematic of the same valve type, showing an idealization of the unsteady flow processes that may occur.

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