Design pressure protection

The following general guidelines are used to design pressure protection from fire ... 

Heat input to a vessel containing only gas is different from a liquid wet vessel, and the following procedure is normally used to design pressure protection of a gas-filled vessel ... 

Design for overpressure protection in most cases consists of providing pressure-rehef devices sized to handle the calcidated relieving rates necessary to prevent emergency pressures from rising above the design pressure (plus allowable accumulation). 

As an alternative means of protection, it is economical in some cases to specify an increased equipment design pressure which will withstand the maximum pressure that can be generated, without reheving any contained fluids. Also, in some cases, the cost of the collection system can be reduced by specifying higher design pressures which will permit a higher back pressure in the collection system. 

In some cases where the ASME Code woidd not require pressure relief protection, the 1.5 Times Design Pressure Rule is apphcable. This rule is stated as follows Equipment may be considered to be adequately protected against overpressure from certain low-probability situations if the pressure does not exceed 1.5 times design pressure. This criterion has been selected since it generally does not exceed yield stress, and most Ukety would not occur more frequently than a hydrostatic test. Thus, it will protect against the possibility of a catastrophic failure. This rule is applied in special situations which have a low probability of occurrence but which cannot be completely ruled out. 

Pump and Downstream Equipment - A PR valve is required for a pump when the shutoff pressure of the pump is greater than the design pressure of the discharge piping, downstream equipment, or pump casing. Positive displacement pumps normally require such protection, while in most cases centrifugal pumps do not. 

If there is a bypass around the control valve, downstream equipment must be protected so that its pressure would not exceed the 1.5 Times Design Pressure Rule, considering that the control valve is in the wide open position, and the bypass 50% open. 

Car-Sealed Closed Valve - In certain cases it may be advantageous to use car sealed closed valves, such as in a bypass around a fuel gas control valve used for furnace flameout protection. The bypass is provided so that the automatic shutdown system can be periodically checked for operation. Where CSC valves are used for other purposes, they are also limited to appUcations where inadvertent opening of the CSC valve would not overpressure the equipment by more than 1.5 times design pressure. 

The header is rated the same as the highest pressure rated equipment connected to it or it is fitted with a safety valve if designed for a lower pressure rating. Sections of the header, separated by check valves, may be designed for different pressure ratings, but safety valve protection is still required for the lower-rated sections, unless the header cannot be overpressured to more than 1.5 times the design pressure. 

If any equipment having a design pressure lower than the maximum output pressure of the compressor is used, or if an increase of pressure above normal operating pressure will cause a malfunction, it shall be protected against overpressure by suitable means. 

A valve designed to protect water system components from excess pressure. 

A pressure relief system must be designed to protect the beverage bottle from overpressure. The relief device will be installed in the C02 line where it enters the beverage container. 

The reactor system in a pilot plant contains stock tanks that are 24 in in diameter and 36 in high. A relief system must be designed to protect the vessel in the event of fire exposure. The vessel contains a flammable polymer material. What rupture disc diameter is required to relieve the vessel properly Assume a discharge pressure of 10 psig. The molecular weight of the liquid is 162.2, its boiling point is 673°R, the heat of vaporization is 92.4 Btu/lb, and the heat capacity ratio of the vapor is 1.30. 

Dispersion, Flaring, Scrubbing, and Containment An example of an overpressure protection system designed to reduce emissions to the atmosphere and at the same time provide adequate protection to the equipment has been described [234]. The equipment indicated is used for the manufacture of ethylene-vinyl acetate-vinyl chloride polymer emulsions. The design pressures are up to 100 bar. 

If it is necessary to locate equipment with explosion vents inside buildings, vent ducts should be used to direct vented material from the equipment to the outdoors. Vent ducts will significantly increase the pressure development in the equipment during venting. They require at least the same cross section as the vent area and the same design pressure as the protected equipment. 

For a liquid thermal expansion relief device that protects only a blocked-in portion of a piping system, the set pressure shall not exceed the lesser of the system test pressure or 120% of design pressure. 

Fire water mains should be designed to handle the maximum pressures developed by fire water pumps. Systems operating at pressures over 150 psi (1,034 kPa) are discouraged, as this would exceed the normal design pressures for most fire protection assets such as monitors, hydrants, etc. 

Thus, in order to adequately protect the equipment , the pressure, relief system should limit the pressure in the. reactor (or an associated item of equipment if it has a lower design pressure) to its maximum accumulated pressure. 

Nowadays, pressures and flow in the process industry are controlled by electronic process systems and highly sophisticated instrumentation devices. Almost all control systems are powered by an outside power source (electric, pneumatic, hydraulic). The law requires that when everything fails regardless of the built-in redundancies, there is still an independent working device powered only by the medium it protects. This is the function of the SRV, which, when everything else works correctly in the system, should never have to work. However, practice proves the contrary, and there are a variety of incidents which will allow the system pressure to exceed the design pressure. 

API has, however, long used the two-thirds rule to identify tube rupture scenarios. This rule states that tube rupture protection is not required when the ratio of the low pressure to high pressure side design pressure is greater than two-thirds. 

So among specialists there are apparently different interpretations or understandings on what API 520 and the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, are really saying, while it is these organizations which are setting the codes that determine how pressure vessels are to be designed and protected. Remember that these codes are law and must be followed. 

The PED, as does the ASME VIII, accepts that the SRV is set at the MAP or PS that is the exact equivalent to MAWP in the ASME code. Also in PED, MAP or PS are the design pressure of the weakest component of the equipment used in a pressure vessel, which needs protecting from potential overpressures. This pressure vessel may be an assembly of different components - pipes, flanges, nozzles, shells, and so on - and each may have a different design pressure. It establishes the limit of the pressure vessel for very short and exceptional increases of pressure above MAP, specifically to enable the SRV to operate properly and reach its rated capacity. This exceptional increase is called accumulation. So accumulation is specific to the individual pressure vessel and does not relate to the SRV. The pressure increase of the SRV to nominal flow is called overpressure. 

An SRV is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP) (Section 3.6). 

ASME Section I SRVs are devices designed to protect power boilers during an overpressure event. Only the U.S. code addresses this sizing separately. PED, on the other hand, makes no distinction between fired and unftred pressure vessels and the method as per Section 8.3 can be used. Here we give the calculation only in metric units. 

If the pressure in the dry flare header exceeds the 3% allowed by most codes, the SRVs connected to it may then open at a pressure higher than what is allowed by this code or law, or even at a pressure that would exceed the acceptable tolerance on the design pressure of the protected equipment (Figure 11.3). 

---