what are pressure relief valves?

INTRODUCTION

Protection against over pressure is one of the most important design tasks in the chemical, petrochemical, oil, and gas industries.pressure relief valves are such instruments that protect the plant form over pressure scenario  The various causes of over pressure fall into two broad categories: fire conditions and process conditions. The purpose of over pressure protection systems is to reduce or eliminate the potential for over pressure-initiated explosions and fires. A pressure relief valves are similar to a fuse in an electrical system.

The Purpose of  Pressure relief valves

Pressure relief valves are commonly installed for one or more of the following reasons:

1. To guarantee the safety of operating personnel
2. To prevent the destruction of capital investment as a result of overpressure
3. To conserve process material from loss during and after an overpressure-related accident
4. To minimize unit downtime caused by over pressure
5. To comply with local, state, national, and other court enforceable regulations
6. To avoid civil suits resulting from property or personal damage external to the plant caused by over pressure By designing and installing reliable over pressure protection systems, the plant will not only obtain favorable insurance treatment, it will minimize pollution (primarily air pollution) by preventing the discharge of over pressure vapors.

CAUSES OF OVERPRESSURE

Overpressure can be caused by fire and by nonfire process causes. In the second category, there can be many potential causes. These will be discussed after the treatment of fire protection that follows in the next paragraph. The potential nonfire causes of overpressure include the following:

1. Utility failures, which can be the failure of electric power, instrument air, steam, coolant, or fuel
2. Thermal expansion
3. Blocked outlets
4. Valve or process control failure
5. Equipment failure
6. Runaway chemical reaction
7. Human error It should be emphasized that part of the goal of a safe plant design is the goal of minimizing the opportunities for human error.

Working of  Pressure relief valves

As it is shown in Figure , the conventional  Pressure relief valve is a force balance device that is held closed by a spring when the inlet pressure is below its set pressure. When the set pressure is reached, the upward force overcomes that of the spring, and the valve opens. When the inlet pressure drops below the set pressure by some percentage (this difference is called blowdown), the valve recloses. The housing of the spring is vented to the outlet of the  Pressure relief valves, and therefore the operation of the valve is directly affected by the backpressure

The  Pressure relief valve inlet incorporates a valve seat with a disc for full closure of the inlet port. The disc is usually spring loaded, and the spring force is applied directly on the disc by means of a stem.

To see the animation of working of    pressure relief valves

To know about Pressure Detectors