This article is about Depressuring System and Vapor Depressurization in Process Vessel of plants and oil and gas industry. Here main article is API RP 521 Section 3.19.
What is Depressuring System?
A depressuring system, used in the oil and gas industry, is a safety mechanism that reduces the pressure inside process vessels during abnormal conditions. It helps prevent equipment failure and potential hazards by rapidly releasing pressurized fluids or gases from the vessel.
This controlled release is achieved through depressurization valves and relief valves, which discharge the excess fluids to a designated vent or flare system. The primary purpose of a depressuring system is to maintain safe operating conditions by lowering the pressure within the vessel during emergencies or predefined scenarios.
If you encounter any of the following conditions, it is recommended to install a vapor depressuring system on the equipment:
- The operating pressure is above 1800 kPa (ga).
- The process equipment contains more than 2 tons of liquid (C4 or more volatile) under normal conditions.
To ensure effective protection, depressuring valves should be located near the equipment and operated from the control room or a remote accessible location. The valves, electric motor, and relevant supply lines within the fire area should be fireproofed.
The initial pressure for depressuring is typically taken as the operating pressure, but it is important to consider the equipment’s design pressure based on operating practices and operator response time. The pressure should be reduced to 50% of the vessel design pressure within 15 minutes, taking into account factors such as vapor generation from liquid due to fire heat input, density changes of the vapor during pressure reduction, and liquid flashing during pressure reduction.
The depressuring valves should be spring-loaded, pneumatic, and diaphragm-operated without a valve positioner. They should have tight shut-off and failure open functions. The minimum size of the depressuring valves should be DN 25 (1 inch), and they should not be provided with a hand wheel. Locked-open block valves should be used for isolation, and locked-closed bypass valves with steam tracing should be provided at grade.
To calculate the total vapor load for a depressuring system, refer to the method outlined in API RP 521 Section 3.19.
Read Also: Alternate Means of Process Overpressure Protection other than Pressure Relief System
Vapor Depressurization or Depressuring in Process Vessel
1. General
1.1 Vapor depressuring is a fire protection method that is used to reduce pressure by voluntary, rapid removal of vapors from pressure vessels, if there is a fire. This method is used to reduce pressure in pressure vessels that would be weakened by excessive heating caused by fire or excessive temperature due to other causes.
It may also be used as a temporary operating control during startups, shutdowns and short-term upsets by reducing pressure to prevent a pressure relief valve discharge. However, vapor depressuring shall not be used as a permanent operating control in lieu of correcting the causes of a unit upset.
1.2 Vapor depressuring requirements for vessels shall be as follows:
a. Vessels shall be depressured to 690 kPa gauge (100 psig) or to 50 percent of the design
pressure, whichever is less. The maximum time allowed to depressure a vessel or system shall be two minutes per 3 mm (1/8 in) of vessel wall thickness, but shall not be less than six minutes. Depressuring time shall not exceed 15 minutes, except with Company approval. For other exceptions, See Section 9.1.1.
b. Vapor depressuring may be impractical when the vessel design pressure is less than 690 kPa
gauge (100 psig), because valves and piping can become unreasonably large and costly. It is also
impractical when the vapor depressuring load governs the size of pressure relief and flare headers.
When vapor depressuring is not practical, vessels may be insulated to reduce the vapor depressuring load or they may be protected by other means, for example, water sprays. The use of either of these alternatives requires SABIC approval.
2. Depressuring Flowrate
To calculate the vapor flowrate that is needed to accomplish depressuring, the maximum expected operating pressure of the vessels under consideration shall be used as the initial pressure and the pressure specified in Section 9.1.1 as the final pressure. When estimating flows for sizing depressuring valves and piping, consideration shall be given to:
a. The effect of the initial depressuring rate on the closed pressure relief system and flare.
b. The average depressuring rate used to determine depressuring time.
c. The variations in temperature, pressure, vapor composition and possible liquid entrainment during the depressuring period on downstream flare system components.
d. The effect of the initial depressuring rate on upstream process equipment.
3. Depressuring Valves
3.1 Types of Valves
Vapor depressuring valves may be pneumatically or hydraulically actuated or manually operated with remote indicators to show valve position. Pneumatically or hydraulically operated valves shall open on accidental actuating fluid failure, with provision to maintain pressure on the diaphragm or hydraulic cylinder in the event of accidental actuating fluid failure. When pneumatically or hydraulically operated control valves are used, they shall be of the tight shutoff type. Block valves that are car sealed open shall be provided to facilitate depressuring valve maintenance.
3.2 Valve Arrangement.
Vapor depressuring valves are typically arranged as follows:
a. Normally, a pneumatically or hydraulically operated depressuring valve is installed in a bypass
around the pressure relief valve. When depressuring valves are used in this location, they shall be
operated from the control room or from a remote, accessible location at grade level outside the fire
area that contains the vessel or the train of process vessels protected by the valve. The valve, its
pneumatic or hydraulic operator and the portion of supply lines located within the expected fire area
shall be fireproofed. The remaining supply lines shall be designed and installed to withstand the heat of the fire for 30 minutes.
b. Pneumatically or hydraulically operated vapor depressuring valves may be placed outside the
fire area when the fireproofing requirements for valves, operators and their supply lines inside the fire area are not economical.
c. Manually operated vapor depressuring valves, if selected, shall be installed in a safe, accessible
location outside the fire area that contains the vessel or train of process vessels protected by the
valve.
d. On towers or vessels containing demister pads or packing, the depressuring valve shall be
located such that dislodgement of the demister pad or packing would not obstruct the valve or
mechanical reinforcement is provided to prevent dislodgement.
3.3 Sizing
a. In sizing depressuring valves, it shall be assumed that heater burners are shut off, reboilers are
shut down and normal flow in the vessel has ceased. Calculate the depressuring load using the
method given in API RP 521. Vapor depressuring valves shall not exceed the capacity of the closed
pressure relief system and flare. Alternatively, they may be programmed to depressure the required
load at a uniform flowrate that is lower than the capacity of the closed pressure relief system and flare throughout the required time period. The second procedure will prevent overloading of the closed pressure relief system and the flare or damage to upstream equipment.
b. When a single depressuring valve serving more than one vessel is sized, equipment other than
the vapor depressuring valve (including control valves) shall not restrict the required depressuring flow. A single fractionating column and its associated equipment or other minor groups of equipment
fall into this category.
c. Depressuring valves are not intended to meet the requirements of the ASME Boiler and Pressure
Vessel Code. These valves provide protection in cases where a vessel might rupture because of
excessive heat from fire or from exothermic reactions before the pressure relief valve actuates.
4. Disposal of Discharge
Vapor depressuring valves shall discharge into a closed pressure relief flare or vent system. This system is preferred because fluid from depressuring valves is likely to be condensable at the end of a sustained release.
5. Other Considerations
5.1 Flare System
It is usually not necessary to size the closed pressure relief system and flare for simultaneous release of pressure relief valves and depressuring valves.
5.2 Pressure Relief Valves
The pressure profile in the closed pressure relief system shall be examined under depressuring loads to ensure that the outlet flanges of pressure relief valves are not overpressured.
5.3 Pressure Vessels
Pressure vessels of copper, aluminum or other materials that are not sufficiently fire resistant shall have protection against fire exposure (for example, external insulation), in addition to having a vapor depressuring system.
Read Also: Pressure Relief System General Requirements in Process Industry
Selection and Design Requirements for Pressure Relief and Vapor Depressuring System
FAQs about Vapor Depressurization
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Why is a Vapor Depressurization necessary?
A depressuring system is necessary to maintain safe operating conditions by rapidly reducing the pressure inside process vessels during emergencies or predefined scenarios. It helps prevent overpressure, which can lead to equipment damage or even explosions.
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How does a Vapor Depressurization system work?
A depressuring system works by opening depressurization valves and relief valves to release pressurized fluids or gases from the vessel. This controlled release helps lower the pressure inside the vessel to a safe level. The released fluids are discharged to a designated vent or flare system.
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When is a Vapor Depressurization system activated?
A depressuring system is activated when certain conditions are met, such as when the operating pressure exceeds a predetermined limit or when the process equipment contains a significant amount of volatile liquids. It is designed to activate automatically or can be manually activated from a control room or a remote accessible location.
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What are the key components of a Vapor Depressurization system?
The key components of a depressuring system include depressurization valves, relief valves, vent or flare systems, and associated instrumentation. These components work together to safely and efficiently reduce the pressure inside process vessels during abnormal conditions.