During the operation of any system in pipelines, due to a malfunction, high pressure can occur, which can lead to the destruction of equipment. To protect the units, a safety valve is installed in the system, which, if the pressure in the pipelines rises above the nominal value, releases the excess working medium, and the nominal pressure is restored in the system.
The principle of operation of safety valves
The high pressure safety device is installed in pipelines and vessels under pressure. Depending on the purpose, the design of safety elements may be different, but the scheme and principle of operation are the same for all.
During normal operation of the equipment, the seat is closed by the spring mechanism and has no effect on the system. The valve setting pressure is equal to the operating pressure in the system. When the pressure in the pipelines exceeds the force of the spring element closing the hole, the seat will open and the conductive medium will be ejected. As a result, the pressure will drop and the seat will close again.
The working medium can be different, water, steam, gaseous fractions, lubricating and hydraulic oils and petroleum products. Therefore, excess pressure can also be discharged to the atmosphere, for example, steam, air, water, or back to the drain line for aggressive materials.
The safety valve begins to open slightly to relieve pressure at a pressure of 3% below the nominal. The discharge of the working medium is initially small, but if this is not enough and the pressure increases, then the saddle will open access to the drain at 110–115% of the working one.
After the pressure drops, the seat will block access, and the system will continue to work in normal mode.
Application of safety devices
To ensure safe operation, the safety fitting is an essential element of any pressurized system.
Depending on the purpose, they can be installed in the following places:
- Hot water supply and heating. The safety valve for the heating system is installed on the pipelines after the supply pump. Since hot water is corrosive, the discharge must be directed to a safe place, usually a sewer. At high costs, the number of units can reach up to 2 or more.
- A plumbing fitting for a cold water pipeline is installed in drinking water supply pipelines. Discharge is made directly to the ground.
- Hydraulic system. Hydraulic oils are used as the working fluid. The hydraulic system serves to drive the working mechanisms: hydraulic motors, hydraulic cylinders. The safety fitting is installed on pipes, or may be part of a pump or control valve. The oil is discharged into the drain line.
- Gas supply pipelines. Increasing the pressure in the pipes can lead to an emergency — this is the separation of the flame from the burners, the accumulation of excess gas and an explosion in the room. Therefore, the fittings are installed immediately after the pressure regulator, and the discharge is made to the atmosphere.
- Air system, compressors. The safety device is installed in the compressor casing and discharged to the atmosphere.
Types of protective mechanisms
In industry, various designs of pipe protection devices are used, depending on their installation location and the conductive medium. These can be spring, lever, diaphragm, two-position and other types of safety valves used.
Let us consider in more detail their device and principle of operation.
Spring safety valve
The most widely used safety valve, especially for heating systems. Its main advantages are simplicity of design and the ability to easily adjust to the working pressure in the system. There are the following types of protective spring mechanisms:
- direct action — the device is triggered by direct exposure to the working environment;
- indirect action — the overpressure is discharged by an external control, for example, an electrical signal;
- two-position units — the opening of the fitting occurs immediately abruptly at full speed, after the pressure is released, the fitting closes abruptly, which can cause self-oscillations of the seat and vibration of the mechanism.
Lever type safety mechanism
The mechanism of this fitting consists of a lever connected to a spool that closes the outlet port. A load is suspended on the lever, and, depending on the weight of the load and the place of attachment on the lever, the mechanism setting is adjusted.
The load is fixed on the lever against accidental displacement and setting changes. The disadvantage of this design is its bulkiness, therefore, the use of such mechanisms is carried out in systems with a large pipe diameter, more than 50 mm.
Diaphragm safety valve
The main element of this design is the membrane. The principle of operation is that when an emergency pressure occurs in the pipes, the membrane breaks and the working medium is discharged. Membrane devices are easy to manufacture, hermetically sealed and respond quickly.
Important! They have a significant drawback, after each operation the membrane must be changed to a new one. Therefore, a conventional fitting is always placed next to such a device.
Depending on the design, such devices can be with a bursting disc and with a slamming disc:
- Rupture disc mechanisms are used in systems with liquid and gaseous working media. The shape of the membrane is flat or dome-shaped, with a sharp change in pressure above the working device, the device is triggered and the membrane is destroyed.
- Flapping diaphragm mechanisms are used most often in systems with a gaseous working medium. The membrane is made of rubber fabric and protects the pipes from both pressure buildup and dangerous pressure drops. To do this, knives are located above and below the membrane. When the pressure changes, the membrane bends, and when, with a sharp drop in the system, it touches the knives, the shell is cut. In this case, depending on the magnitude of the pressure, either a discharge or a replenishment from an additional tank occurs.
Installation and installation
To ensure the safe operation of gas and water pipelines, when installing protective units, it is recommended to comply with the following requirements:
- Shut-off valves and filters must not be installed on pipes suitable for the protective mechanism and pressure relief pipelines.
- The axis of the spring fitting must be installed vertically. The unit must be freely accessible for replacement and adjustment.
- The lever-type safety lever must be horizontal when installed.
- A pressure gauge must be installed in front of the fitting to control the pressure.
- Depending on the working medium, the discharge pipe can be directed: to the atmosphere for steam and gaseous non-aggressive media. For hot water, the drain must be directed to a special container, or back into the system. If there is an aggressive medium in the pipes, then the discharge should only be drained into a closed tank.
- The length of the pipe from the equipment to the installation of the protective unit must be kept to a minimum. In this section, it is not allowed to connect branches for the selection of the working medium.
- The safety valve for the heating system can be installed in several places on the pipeline, while the flow rate of the working medium through the pipeline must be 1.25 times greater than the total flow rate of the installed devices.
- Before putting the system into operation, it is imperative to check the correct setting of the safety valve, as well as its ability to return to its original position and ensure tightness.
The durability of the protective mechanisms depends on compliance with all conditions of technical operation. During the operation of the protective unit due to wear of the main parts, the following defects may occur:
- Seat leakage. The defect may occur as a result of the ingress of metal chips and the presence of scratches on the saddle. The defect is eliminated by grinding the seat or replacing it with a similar fitting.
- Understated opening pressure of the device due to the loss of its elastic properties by the valve spring or misadjustment of the setting. To eliminate the failure, it is necessary to replace the spring or the fitting itself, adjust the pressure, check in operation and put a seal.
- If it is necessary to replace the unit for repair, then a plug or valve cannot be temporarily placed in its place. It is necessary, for the safety of the object, to first select a valve with exactly the same characteristics, and only install it instead of the removed one.
- If there is a pulsation during operation, rapid opening and closing of the shutter of the device, then such a defect can cause unwanted vibration of the pipelines, which can lead to their deformation. The reason may be a mismatch between the dimensions of the sections of the main pipeline and the pipeline connected to the unit. To eliminate the defect, it is necessary to install pipes of the same cross section during installation.