The relief header is a system of piping connecting the outlets of all the relief valves into a common pipe or header that goes to the relief scrubber and then out the vent as shown in Figure 13-11. There are some general rules of thumb useful for sizing relief piping. The goal is to make the piping big enough so it doesn’t restrict the flow of the relieving fluids and impose excessively high back-pressure on the relief valves flowing into it. However, designing very large diameter piping costs money and uses up valuable space on the platform.
The pressure in the relief piping is usually equal to atmospheric pressure as long as no relief valve is relieving. There is a common misconception that, since the relief piping is open to the atmosphere, the pressure remains atmospheric. However, when a relief valve goes off, the pressure in the relief piping increases due to the friction of the gas moving through the piping. There is pressure drop in relief piping, just like any other piping. The pressure at the vent tip is atmospheric, so the pressure at the relief valve outlet must be higher for the fluid to flow out. Generally, relief piping is designed for much higher velocity than process piping because the service is intermittent; therefore erosional velocity can be ignored, and higher noise levels are acceptable. The result of the higher velocity is much higher-pressure drops than in process piping.
The relief header is designed using a reasonable combination of simultaneous relieving cases. It is not reasonable to assume that all relief valves are relieving at design rates at the same time. It is not necessary to assume multiple failures happening at the same time, unless a single event can cause several valves to relieve at the same time. An example of this would be that a sales pipeline shutdown could cause the compressor to blow down plus the production separator relieving.
he back-pressure at the outlet of a relief valve depends on the relieving rate and the length and size of the outlet piping. It can’t be determined from the vessel pressure and PSV orifice size because the flow is choked and is independent of the outlet pressure as long as this pressure is less than 50% of the set pressure. The two known quantities for relief piping back-pressure calculations are the flow rate, which is controlled by the PSV size, and the pressure at the outlet of the vent tip—atmospheric pressure. Unlike most other pressure calculations, relief systems require starting at the outlet and working backward to determine what pressure is required to push the gas out the tip. Once steady state is reached, no gas is accumulating in the header. All the gas that exits the relief valve must exit the vent tip. In order for the relief valve to operate at design flow rates, it should be in critical flow, so back-pressure must be less than 50% of the set pressure. We normally assume the relief valve is in critical flow and size the outlet piping to ensure the back-pressure is low enough. If the back-pressure is too high, then relief valves have reduced relieving capacity. If the back-pressure between the vent scrubber and the vent outlet is too low, then the low pressure, high volume gas will require an excessively large vent scrubber.