Safety valve
A safety valve is a valve mechanism for the automatic release of a gas from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits. They are often called by more specific names such as pressure relief valves, T&P valves, or temperature and pressure relief valves.
Safety valves were first used on steam boilers during the industrial revolution. Early boilers without them were prone to accidental explosion when the operator allowed the pressure to become too high, either deliberately or through incompetence.
The earliest and simplest safety valve used a weight to hold the pressure of the steam, but these were easily tampered with or accidentally released. On the Stockton and Darlington Railway, the safety valve tended to go off when the engine hit a bump in the track. A better valve used a spring to contain the steam pressure, but these (based on Salter spring balances) could still be screwed down to increase the pressure beyond design limits. In 1856 John Ramsbottom invented a tamper-proof spring safety valve which became universal on railways.
Safety valves also evolved to protect equipment such as pressure vessels and heat exchangers. The two general types of protection encountered in industry are thermal protection and flow protection.
Thermal relief valves are generally characterized by the relatively small size of the safety valve necessary to provide protection from thermal expansion pressure increases in liquid-packed vessels. As most liquids are considered fairly incompressible, it takes a relatively small amount of fluid discharged through the relief valve to provide an adequate level of protection.
Flow protection is characterized by safety valves that are considerably larger than those mounted in thermal protection. They are generally sized for use in situations where significant quantities of gas or high volumes of liquid must be quickly discharged in order to protect the integrity of the vessel or pipeline.
They are required on water heaters, where they prevent disaster in certain configurations in the event a thermostat should fail. There are still occasional, spectacular failures of older water heaters that lack this equipment. Houses can be levelled by the force of the blast.
Pressure cookers are pots for cooking with a pressure proof lid. Cooking at pressure allows the temperature to rise above the boiling point of water (100 degrees Celsius at sea level ) which speeds up the cooking and makes the cooking more thorough.
Pressure cookers usually have two safety valves. One in a hole upon which a weight sits. The other is a sealed rubber grommet which is ejected in a controlled explosion if the first valve gets blocked.
The term safety valve is also used metaphorically.
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May 4th, 2008 at 11:10 pm
For some unknown reason, relief valves are often considered maintenance free
For some unknown reason, relief valves are often considered maintenance free. It is assumed by some if the valve has not relieved in a year or more that everything is satisfactory and no reason should exist to investigate and determine if any maintenance is necessary. All type valves, spring-loaded; diaphragm, integral pilot and others should be examined and tested periodically to determine if maintenance is required. Usually visual examination of the component parts is sufficient; however the valve must be re-assembled and set before returning to operation. The use of some types of relief valves makes this field maintenance an impossibility as lapping compounds are required to grind the seat, and field-testing and adjustments are practically impossible. Valves that cannot be field maintained must be removed and carried to a test shop to ascertain if any maintenance is required. Maintenance of relief valves should coincide with prescribed maintenance of other associated equipment, such as motor valves and controllers.
Relief valves should be field tested
Relief valves should be field tested periodically to determine if setting are correct. Some personnel pass by relief valves daily and in most cases assume they are protected. This might go on for years without testing the valve and yet the one time in many that the relief valve is called upon to function, corrosion or internal malfunction will not permit the valve to open and a dangerous overpressure occurs.
Testing of almost any valve can be performed in the field by use of nitrogen or a high-pressure connection on upstream side of meter station and a long high-pressure hose to the relief valve. It is necessary that a block valve be installed below the relief valve. Attach the high-pressure hose to the connection and increase the pressure until the valve relieves. Naturally it will be necessary to read a pressure gage on another connection within the spool.
If a block valve is not installed, to ascertain if the relief will function, even though the pressure setting cannot be determined, lift the lever, or in the case of U. S. Industries Relief Valve, lift the stem on the relief control. This causes the valve to go through a cycle, and at least you will know that all the internal parts are free. On installations of this type, take advantage of opportunities to test the valve when pressure is off the stem.
May 4th, 2008 at 11:11 pm
he excellent pilot valve mechanism provides rapid response under any condition and prevents inlet pressure from rising.
The unique closure mechanism maintains complete water shutoff in the fully closed condition.
The pilot valve does not have any minute, fixed restriction passage like a needle valve, and is therefore clog-resistant.
And, even if clogging by foreign material occurs, the built-in self-cleaning mechanism causes the clogged valve part to open automatically to remove the foreign material.
Due to simple construction, troublesome adjustments are unnecessary. Being excellent in durability and practically maintenance-free, using this valve reduces maintenance costs.
1. When inlet pressure (primary pressure) P1 rises above the set pressure, the pilot valve operates and the main valve is opened to release the excess pressure to the outlet side and maintain the inlet pressure at the set pressure.
2. Under normal conditions, pilot A valve is open and pilot B valve is closed so that inlet pressure P1 is transmitted to the top of the main valve to keep the main valve closed.
When inlet pressure P1 exceeds the set pressure, pilot A valve closes and pilot B valve opens to reduce the pressure acting on the top of the main valve, thus opening the main valve and preventing the inlet pressure from rising.
3. The set pressure can be finely adjusted with the coil spring of the pilot valve