1. A SAFETY VALVE is used to relieve pressure in an over pressure situation. It is the last line of defence to prevent a catastrophe. The following text explains the working of safety valve briefly taking an example of Makestar 66 series of safety valve. Click here for a picture of the safety valve, the figures in brackets are to be referred back to this picture. The operation is explained below with reference to the picture. Read the manual for complete information. Click here to download the operating instructions
2. The process pressure is overcome with the help of a spring(15) load. One end of the nozzle (04) is connected to the inlet flange(25) in the bottom and the other end opens to a chamber which has an outlet flange to the side. In normal operating condition, the Disc(05) sits on the nozzle and tightly closes shut so that no process pressure escapes towards the outlet flange. The disc is screwed to a Disc holder also called piston (08)
3. when the process pressure exerts more force than the spring load, the valve guide(09) lifts, thereby lifting the disc from the nozzle and the process pressure escapes through the outlet. If the excess pressure is relieved through the outlet slowly, it is a pressure relief valve, whereas if the entire plant capacity is flared during a pop-up , it is a pressure safety valve.
4. The Spring and nozzle are kind of heart of the safety valve. The spring action is obvious, you have an adjustable screw(17) with a lock nut (18) on top and the bottom portion of this screw rests on a spring washer (16), the spring washer sits on top of the spring. The spring is loaded with the tightening of the lock nut. The nozzle is carefully designed and various types of nozzles are available based on API standards.
5. The nozzle is designed according to API-526 standards and the design equation of the cross section area of the nozzle is given in this manual. Click here to download a manual for crosby safety valve and go to page no. 31(page 6-3) for explanation. The cross section area is designed such that the entire volume of the plant capacity has to be relieved in case of safety valve actuation. Therefore, the mass flow rate in Kg/hr is important in this equation. The mass flow rate can be calculated by multiplying the handling capacity of plant in sm3/hr and the base density of natural gas. (density at P=1.01325 bar and T=15 deg C ), in our case, it is 0.7
6. valve relieves for a specified back pressure at the output flange. If back pressure exceeds then PSV with bellows is required.
7. If valve chatters in rapid sequence, the blow down ring (06) may be too high, adjust blow down ring to lower setting as per procedure.
8. If valve opens at set pressure, but doesn't close within blow down range, then, blow down ring may be high, adjust it to lower
9. If popping sound is not proper, blow down ring is not at proper position, raise the blow down ring as per procedure.
10. I could not find any specific design guidelines on inlet and outlet pipe size, but after going through datasheets, it is found that the inlet piping is about 2 times the calculated dia of Nozzle. and the outlet pipe size is inlet pipe size + 2 inches as a matter of practice. The outlet pipe size should be more than the inlet pipe size.
11. Installation as per API RP 520 part 2. Maintenance as per API RP 576
12. This youtube video provide good information about operation and maintenance of safety valves.
13. Download a sample datasheet here and one can go through the various points with respect to the note above.