Pressure Relief Valves

PRESSURE RELIEF VALVES
Chemical Engineering Lab I
Group Project
Group II
Members: Kuntu Satterwhite, Janique Ricketts, Teresia Kiangi
Report Submitted: 14th April, 2000






















Question: Discuss the importance of Relief Valves in the unit operations in detail, and give the design criteria/ parameters/ models available equations in the literature. Support your work by giving a typical example from the literature.

INTRODUCTION
Since the inception of pressurized systems to produce and transmit energy, it has been a constant goal to find safe, dependable means of relieving pressure. The simple, inexpensive, spring loaded relief valves has been manufactured for over a hundred and fifty years, performing its job with varying degrees of reliability over that time period.
In the Unites States alone, boiler explosions caused a significant loss of life reaching a peak with 400 such explosions in 1904. With the adoption of the ASME Pressure and Vessel Code in 1911, boiler explosions declined to where, after 1973 they are non-existent. Over the same period, operating pressures for most systems were increased, in the case of boiler pressures increases were from 200 to 3000 psi and above.
Pressure relief valves are used to safely contain pressurized gases and liquids. Throughout the years there has been an increased demand for safety in industrial and other related operating practices. These valves are one of the many devices that have been created in conjunction with standardized design and operating practices, for the provision of this very protection. In the past, vessels that now would obviously require relief valves were heated often resulting in drastic increase in pressure within the vessel. This resulted in an extremely dangerous outcome physically for all the parties involved.
Pressure valves may be classified into two main categories namely reclosing and non-reclosing. Reclosing devices are the more common of these two groups. They are designed in such a manner that they reclose securely once safe pressure levels have been restored. The second type, the non-reclosing pressure valve, does not reclose nor vent all the fluid built up, for example fusible plugs.
Pressure relief valves are relatively simple pieces of equipment. The operation of these devices involves a spring-loaded disk resting on a seat. When the pressure is low the force on the disk is less than the spring force on the disk and the valve remains closed. The pressure force increases causing a drop in the force that seals the disk on the seat. Once the valve is raised a larger disk area is available for the fluid to flow through providing a lifting force which is directly correlated. The valve will then continue to rise until the compressed spring generates enough force to stop it.
Once the valve has risen the fluid will then flow through the valve. This fluid may be either gas or liquid. Due to specific passages in the valve the fluid will then exert a dynamic force on the valve which arise as a result of momentum changes and drag. This causes the disk to raise even higher and with a higher elevation a higher low is produced. The maximum height the valve can attain is classified as the throat area. When the valve is open to an area equal to the throat area that is referred to as the quotocurtain area. At this point the system is in steady state. If the disk continues to rise this will have no effect on the flow and the system will be said to be in a steady state operation.
The pressure inside the vessel decreases as the fluid flows through the valve. The disk is held open by static pressure as well as dynamic flow forces, which act, on the full area of the disk. In order for the spring forces to overcome the forces exerted by the fluid and reclose the valve the pressure must fall well below the original pressure which caused it to rise in the first place. The pressure at which the disk reestablishes contact with the seat is referred to as the reclosing pressure.


Category: Science