How to Determine Design Pressure

As process engineer dealing with process equipment, we need to determine design pressure. For a vessel, design pressure is called maximum allowable working pressure (MAWP). The MAWP determines the setting pressure of relief valve and must be higher than vessel’s operating pressure. Operating pressure is fixed by process condition.

Table below recommends a minimum differential between operating pressure and MAWP so that the difference between operating pressure and relief valve set pressure provides a sufficient cushion. If the operating pressure is too close to the relief valve setting pressure, small surges in operating pressure could cause the relief valve to activate too soon.

Table 1 – Setting Maximum Allowable Working Pressures

In several projects, I usually design a vessel equipped with pressure safety high sensors (PSHs). Sometimes there are two sensors, which are PSH and PSHH. The sensors will shut in the inflow if higher-than-normal pressure is detected. The differential between the maximum operating pressure and the PSH sensor set pressure should be as indicated in table above. And the relief valve should be set at least 5% or 5 psi, whichever is greater, higher than PSH set pressure.

For example, a vessel operates at 75 psig. Thus, the minimum recommended MAWP for the vessel equipped with PSH sensor would be 105 psig (75 + 25 + 5). The PSH sensor is set at 100 psig and the relief valve is set at 105 psig.

For small vessels, it will be more advantageous to use higher MAWP than it is recommended in table above. It may be possible to increase the MAWP at little or no cost and thus give greater flexibility in the future if process changes.

Be noted that the MAWP of the vessel cannot exceed the MAWP of the nozzles, valves, and pipe connected to the vessel. Table below is summary of ANSI B16.5 pressure rating material group 1.1.

Table 2 – Summary ANSI Pressure Ratings Material Group 1.1

If the minimum MAWP calculated based on Table 1 is closed to one of ANSI MAWP in Table 2, it is common to design pressure vessel to the same MAWP as the ANSI class. Often, a slightly higher MAWP than that calculated from Table 1 is possible at almost no additional cost.

Once a preliminary MAWP is selected from Table 1, it is necessary to calculate wall thickness for the shell and heads of the vessel. The actual wall thickness selected for the shell and heads will be somewhat higher than that calculated as the shells and heads will be formed from readily available plates. Once the actual wall thickness is determined, a new MAWP can be specified for essentially no additional cost (there will be a little increase in cost of vessel testing to the slightly higher pressure).

This concept can be especially significant for a low-pressure vessel where a minimum wall thickness is desired. For example, assume the calculations for a 50-psig MAWP vessel indicate a wall thickness of 0.20 in, and it is decided to use 1/4-in plate. This same plate might be used if a MAWP of 83.3 psig were specified. Thus, by specifying the higher MAWP (83.3 psig), additional operating flexibility is available at essentially no increase in cost. Many operators specify the MAWP based on process conditions in their bids and ask the vessel manufacturers to state the maximum MAWP for which the vessel could be tested and approved.

References:

Arnold, Ken and Maurice Stewart. Surface Production Operations: Design of Oil Handling Systems and Facilities. Elsevier: 2008