Safety valve sizing

The above calculation indicates that the vapour flow rate is 2.63 kg/s and the gas flow rate is 0.4536 kg/s. This can be used to obtain weighted average values of physical properties required for calculating the relief system capacity. 

Assume that the back pressure on the safety, valve will be limited to 10% of the gauge set pressure. (The back pressure required is the sum of the constant and built-up back pressure. If the relief system discharges direct to the atmosphere, then the constant back pressure will be zero.) If the relief pressure is 4.4 barg, the set pressure is 4.4/1.1 = 4.0 barg, and the maximum back pressure is 0.1 x 4.0 = 0.4 barg. Thus 4 

This is true, and so flow through the safety valve will be choked. Equation (A6.4) can then be used to obtain the mass flow capacity per unit area of the safety valve, setting FB equal to 1.0 because the flow is choked. From Table A6.1, for = 1.1, Csv is 6.90x1 O 3. 

The required flow area of the safety valve can now be found using equation (A6.1)  

This can be used to select the required size of safety valve. For the type of safety valve chosen, the next largest valve size has an actual area of 2.36x10 3 m2. The inlet and outlet pipe sizes for this valve are 0.075 m inlet and 0.1 m outlet. 

---

The most difficult task for the safety-valve sizing is the achievement of the maximum flow-rate to be discharged. In case the safety valve is used to save the operation of a pump or compressor, their maximum mass-flow-rate is the basis for the sizing. Much more effort is necessary in systems where vessels, complex piping systems, pumps, and thermal expansion can activate the safety valve. In most cases, some discussions with the relevant authorities are necessary to clarify all assumptions for risks relevant to safety. 

A COMPARISONS BETWEEN THE SAFETY VALVES SIZING FORMULAS1... 

A Comparisons between the safety valves sizing formulas... 

Example 5. Find the safety valve size given IF =... 

Schmidt, J., Peschel, W., and Beune, A. (2009) Experimental and theoretical studies on high pressure safety valves sizing and design supported by numerical calculations (CFD). Chem. Eng. Technol., 32 (2), 252-262. 

What if area wide electrical power fails. (UPS instrumentation power remains) Runaway reaction through loss of agitation. Indicated by agitator motor off, low coolant flow, high reactor pressure, and high reactor temperature. Runaway reaction causes reactor overpressure and loss of containment. Add shortstop and burp reactor to stop runaway. Depressurize reactor - SIS (Pressure safety valve sized for this event). Use LOPA to determine required SIL ... 

The entire inlet line must be at least the size of the PR valve inlet. When multiple safety valves are manifolded on the inlet side, the cross-sectional area of the maitifold piping must be equal to or greater than the sum of all the inlet areas of valves open to the manifold. 

The single contingency which results in the largest accumulation (B-E) of liquid safety valve releases during 30 minutes, regardless of any associated vapor rate. The level at point E is used for pumpout pump sizing. 

We shall first consider the design of elevated flare systems. Sizing of flare systems is a function of maximum allowable back pressure on safety valves and other sources of release into the emergency systems. 

Flare stack sizing and pressure drop is included with considerations of pressure drop through the safety valve headers, blowdown drums, flare headers, seal drum, etc. Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 120 m/s at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will insure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. 

But the outlet nozzle size of the safety valve is 6 in. Schedule 40. Hence, 6 in., will be minimum line size. (This is normal practice.)... 

For example, increasing a valve size or installing a larger pump could result in high pressure in a vessel, thus increasing the risk of a release. Sanders (1993) presents a number of examples of changes affecting the safety of a plant. 

Safety valves and vacuum breakers, per inch of nominal size. 1.0... 

For conventional valves, pressure drop or variations in back pressure should not exceed 10% of set pressure. Because most process safety valves are sized for critical pressure conditions, the piping must accommodate the capacity required for valve relief and not have the pressure at the end of vent or manifold exceed the critical pressure. Designing for pressure 30% to 40% of critical w ith balanced valves, yields smaller pipes yet allows proper functioning of the valve. The discharge line size must not be smaller than the valve discharge. Check the manufacturer for valve performance under particular conditions, especially with balanced valves w hich can handle up to 70% to 80% of set pressure as back pressure. 

Rupture disks are used for the same purpose as safety valves and, in addition, serve to relieve internal explosions in many applications. If the pressure rise can be anticipated, then the volume change corresponding to this change can be calculated by simple gas laws, and the capacity of the disk at the relieving pressure is knowm. The system must be examined and the possible causes of overpressure and their respective relief capacities identified before a reliable size can be determined. See Figure 7-14. 

Depressuring devices may be used, but no credit for their use shall be allowed in sizing safety valves for fire exposure,... 

Hattiangadi, V. S., Sizing Safety Valves for Heat-Sensitive Materials, Chem. Eng, Sept. 7, 1970, p. 96. 

Safety valves are often required on the shell side of exchangers and sometimes on the tube side. These valves may require sizing based upon process reaction, overpressure, etc., or on external fire. For details, see Chapter 7, Vol. 1 on safety-relieving devices. 

Safety-valve vent pipework should be mn on the shortest possible route. Where bends or long mns occur, the pipe size may have to be increased to prevent back pressure on the safety valve during operation. 

Where an isolating valve is installed in the discharge pipework the pipeline on the compressor side of the valve shall be protected by a suitable safety valve. This safety valve must be of sufficient size to pass the full output of the compressor without the pressure rising more than 10 per cent above the maximum allowable working pressure. 

Positive-displacement machines use safety valves. There are ASME standards of materials, sizing, and only ASME stamped valves should be used. The relieving capacity of a given size of safety valve varies materially with the make and design. Care must be taken to assure proper selection. 

Sizing, 451, 453, 455, 459, 462 Sonic flow, 461 Types, illustrations, 411-421 Rupture disk, liquids, 462, 466 Rupture disk/pressure-relief valves combination, 463 Safely relief valve, 400 See Relief valve Safety valve, 400, 434 Safety, vacuum, 343 Scale-up, mixing, 312, 314—316 Design procedure, 316-318 Schedules/summaries Equipment, 30, 31 Lines, 23, 24 Screen particle size, 225 Scrubber, spray, 269, 270 Impingement, 269, 272 Separator applications, liquid particles, 235 Liquid particles, 235 Separator selection, 224, 225 Comparison chart, 230 Efficiency, 231... 

Morley, P. G. (1989a) Chem. Engr., London No. 463 (Aug.) 21. Sizing pressure safety valves for gas duty. 

---