Relief systems include, but are not limited to, relief valves, relief headers, relief drums, and rupture disks. Design basis means documenting how the loads and sizes of the relief system and inlet and outlet sizes were determined. This includes:
- a description of overpressure scenarios considered,
- the scenario that creates the largest load to be relieved,
- the assumptions used, and
- if the device meets a certain code
Relief devices on pressure vessels must conform to ASME codes. Industry codes (e.g., API RP 520) also provide guidance on scenarios that should be considered and on equations for the sizing of devices. Scenarios a facility may need to consider include:
- fire,
- blocked flow,
- control valve failure,
- overheating,
- power outage,
- tube rupture, and
- cooling water failure.
For the two-phase flow, you should review AIChE publications from the Design Institute for Emergency Relief Systems (DIERS).
source for the above information is EPA RMP Q&A
What a “relief valve design: basis SHOULD include:
A relief system is a combination of a pressure relief device and the associated lines and process equipment that is used to safely handle the fluid. The relief design methodology needs to be considered in the correct order as follows:
1. Locate potential relief points.
- On all pressure containing vessels (liquid or gas)
- Blocked in sections of cool liquid lines that could be exposed to heat in any shape or form.
- Discharge sides of positive displacement pumps, compressors, turbines, etc.
- Vessel steam jackets
- Chemical reactors
- Heat exchangers
2. Choose the general type of the relief device.
- Rupture discs
- Spring-operated SRVs
- Pilot-operated SRVs
- A combination of the above
Depending on the specific application, it will be determined later which specific configuration of the above general types we should use in order to get optimal efficiency.
3. Develop the different possible overpressure scenarios for a specific pressure-containing vessel.
a. Description of one specific relief event
b. Usually, each possible relief has more than one relief event and multiple scenarios. For example:
-
- Overfilling
- Fire
- Runaway reaction
- Blocked lines with subsequent expansion
- Developed through a PHA (process hazard analysis)
Overpressure Scenario
1. Control valve on a feed line can be stuck open while the manual outlet valve at the bottom is closed, and the vessel can overfill.
2. The steam regulator to the steam jacket can fail and cause overpressure in the vessel.
3. Coolant system could fail, which can cause a runaway reaction as a result. Then we can start sizing the system for the necessary relief.
- Determining the necessary relief rates.
- Determining the relief vent area.
In any case, ALWAYS use the WORST CASE as the necessary relief flow scenario.
4. Design the complete relief system. A relief system entails more than just installing an SRV or a rupture disc on a pressure vessel; it also includes the following:
- Look for the necessity of a backup relief device(s) – evaluate the necessity for eventual redundancy (possibly for maintenance reasons). This can be two RVs, two rupture discs or a rupture disc in parallel with an RV. It is recommended to stagger the settings slightly, having the RV open first.
- Design the correct piping leading to the relief device(s) – avoid excessive inlet pressure drops.
- The environmental conditioning of the relief devices – can they discharge to the atmosphere or not?
- Design the discharge piping/headers – avoid the unnecessary creation of backpressure on the RV(s), or determine the correct backpressure so it can be taken into account when sizing and selecting the relief device.
- Design a blowdown drum when needed.
- Design the condensers, flare stacks or scrubbers (if any).
OVERPRESSURE RELIEF DEVICES
In the process industry, different types of relief devices are used as primary protection, which can be divided into the following groups of relief devices to ensure overpressure protection:
- Reclosing devices
- Non-reclosing devices
- Combinations of reclosing and non-reclosing devices
Some individual parameters, both technical and economical, drive the choice of the best solution.
Generally speaking, the use of non-reclosing pressure relief devices will offer, in most cases, the lower cost solution but requires that the process is shut down or redirected through alternative safety systems to allow for the replacement of the burst device. Subsequently, non-reclosing pressure relief devices will only be selected as primary relief solutions in cases where the loss of process media or shutdown for repair is tolerated or possible. (i.e., NO toxins or flammables!!!!)
However, selecting non-reclosing devices as a secondary or backup system is a more widely accepted solution. Reclosing devices allow for the continued operation of the process, even when spurious overpressures occur. Consequently, reclosing devices will be preferred for primary relief applications where the long-term opening of the process equipment cannot be tolerated. The potential for leakage, fouling, plugging or icing can, however, render these critical devices sometimes inefficient, and great care is needed in making the correct selection of the device, taking into account both the application and the local codes. This also accounts for the wide variety of valves available on the market; they are all there for a reason.
Reclosing relief devices are mainly safety valves, relief valves, or SRVs (either weight, spring, or pilot operated), whereas the non-reclosing relief devices are bursting disc or buckling pin devices. Combinations of SRVs and bursting discs are also relatively popular as they offer the best of both individual solutions. The most commonly used combination will be a design where the bursting disc device is installed upstream of the safety valve. In such a configuration, the bursting disc device will provide a pressure and chemical seal between the process and the downstream valve, resulting in a better safety factor and reduced operational and maintenance cost (leakage, repair, corrosion, etc.) on the SRV in case of dirty, corrosive or polymerizing fluids. The use of bursting disc devices on the downstream side of safety valves may be considered in cases where corrosion or fouling of the valve trim may be a concern (a common problem in systems using common headers to evacuate process media). In all cases where combinations of bursting disc devices with SRVs are used, measures must be taken to ensure that the space between the valve seat and the bursting disc is kept at atmospheric pressure. Any increase of pressure in this cavity due to, for example, temperature changes, minute pressure leaks, and so forth, will result in a dramatic and uncontrolled change in the opening pressure of the safety system. Also, with this combination, the SRV must be 10% oversized to accommodate the eventual pressure drop over the bursting disc.
Alternatively, in cases where pressure relief cannot be applied due to environmental or process issues, the use of controlled safety pressure relief systems (CSPRS) or safety-related measurement, control, and regulating (SRMCR) devices may be evaluated. They are used primarily on steam systems. Such systems will generally be developed to interact with the process to avoid the occurrence of situations possibly leading to unsafe conditions. Such systems have to be carefully selected, taking into account guidance regarding safety redundancy specified in design documents such as IEC 61508 ‘ Functional safety of electrical/electronic/ programmable electronic safety-related systems’, IEC 61511 and ANSI/ISA S84.01.
Fire case
This case covers the event that the pressure vessel is exposed to external fire, which would cause the system to heat up quickly. Vapors would expand; hence there would be a faster increase in pressure above the system design pressure. In the case of liquid storage, the liquid would flash into vapor, causing a significant pressure increase.
Blocked discharge
With the blocked outlet case, external fire cases are probably one of the most common cases where SRVs are required in the modern process industry. The procedure used for fire sizing sometimes depends on the codes and engineering practices applied in each installation and is determined by the end users. The following sizing procedure, according to API RP 520 Part 1, is the most commonly used. We will here detail the calculations according to the code and will later also give the (conservative) simplified empiric method used by some professionals.