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Tech Notes

APPLICATIONS OF TECHNOLOGY TO LOSS PREVENTION

As the opportunity occurs LCAI will be posting information on technology and applications of engineering principles to Loss Prevention and Process Safety Management. We welcome your comments and suggestions.

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Articles posted on this page include:

VAPOR DEPRESSURING FOR BLEVE PREVENTION

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VAPOR DEPRESSURING FOR BLEVE PROTECTION

Introduction

Fire Exposed Process Receiver PHOTO

Depressuring System Design Requirements

Depressuring Flowrate

Depressuring valves

Sizing

Disposal of Discharge

Other Considerations

Pressure Vessels

Introduction

Vapor depressuring is a fire protection method used to reduce pressure by voluntary, rapid removal of vapors from pressure vessels in case of fire. This method should be used to reduce pressure in pressure vessels that would be weakened by excessive heating caused by fire or excessive temperature due to exothermic reactions. It may also be used as an operating control during startups, shutdowns, and short-term upsets by reducing pressure to prevent a pressure relief valve discharge. However, vapor depressuring should not be used as a permanent operating control in lieu of correcting the causes of a unit upset. Advantages of vapor depressuring are:

(a) It produces an autorefrigeration effect in a pressure vessel, which provides cooling of liquid contained in the vessel.

(b) It eliminates the need for a liquid blow-down system. The liquid retained in the vessel minimizes increased shell temperature on the wetted surfaces.

(c) It provides for depressuring of pressure vessels containing large volumes of volatile flammable liquids that may be exposed to fire. Usually, such vessels or a train of vessels within a process unit can be depressured through a single valve.

FIRE EXPOSED PROCESS RECEIVER

Note line at liquid level

The advantages of vapor depressuring of a process vessel containing a boiling liquid contents is illustrated by the photo. The process vessel depressured during a refinery gas plant fire. In this instance failure of a run down line at the vessel nozzle at the initiation of the incident resulted in rapid depressuring of the vessel. This release of pressure from the vessel aided the control of the following fire since cooling and control of flame impingement on the vessel shell resulting from the subsequent fire prevented vessel failure and release of contents. Such a catastrophic failure is often termed a Boiling Liquid Expanding Vapor Explosion or BLEVE. Effectiveness of the depressuring is often illustrated by a sharp line on the vessel wall which corresponds with the level of liquid remaining in the vessel when exposed to fire.

Depressuring System Design Requirements

Vapor depressuring should be provided using the following criteria:

a) Bare, uninsulated vessels require vapor depressuring when the contain volumes of liquid that can be vaporized when fire occurs.

Only for depressuring purposes and where stainless steel, vinyl-clad galvanized steel, or uncoated galvanized steel jacketing is used, credit may be taken for external insulation on vessels to reduce depressuring loads. However, the insulation should be:

• A hard asbestos-free insulation, suitable for temperatures at least up to 649 ° C (1200 ° F)
• Properly Installed as specified in manufacturers requirements.

The insulation and jacketing is most effective only when both are held in place by stainless steel bands so that they are resistant to dislodgment by fire hose streams. When insulation is not of the type and quality specified herein, the vessels should be treated as bare vessels.

Vessels of the type described in Items (a) and (b) should be depressured to 690 kPa gage (100 psig) or to 50 percent of the design pressure, whichever is smaller. The maximum time allowed to depressure a vessel or system shall be two minutes per 3 mm ( c inch ) of vessel wall thickness but shall not be less than six minutes. Depressuring time should not exceed 15 minutes except with owner approval. For other exceptions, refer to Item (d).

(d) Vapor depressuring may not be practical when the vessel design pressure is less than 690 kPa gage (100 psig) (valves and piping can become unreasonably large and costly) or 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 may be protected by other means such as water sprays (see NFPA 15, Fire Water Sprays). Good engineering judgment is required when investigating the application of vapor depressuring.

Depressuring Flowrate

To calculate the vapor flowrate needed to accomplish depressuring, the maximum expected operating pressure of the vessels under consideration should be used as the initial pressure, and the pressure specified in Item (c) Above as the final pressure. When estimating flows for sizing depressuring valves and piping, consideration shall be given to:

• The effect of the initial depressuring rate on the closed pressure relief system and flare.
• The average depressuring rate used to determine depressuring time.
• The variations in temperature, pressure, vapor composition, and possible liquid entrainment during the depressuring period.

Depressuring Valves

Types of Valves

Vapor depressuring valves may be electric motor operated gate valves, pneumatically operated control valves, or manually operated outside screw and yoke (OS&Y) gate valves. Pneumatically operated valves shall open on air failure, with provision to maintain pressure on the diaphragm in the event of instrument air failure. When pneumatically operated control valves are used, they shall be of the tight shutoff type. Locked-open block valves should be provided to facilitate depressuring valve maintenance.

Valve Location

Requirements for a vapor depressuring valve are as follows:

(a) Normally, an electric motor operated or a pneumatically operated depressuring valve located in a bypass around the pressure relief valve is used. When valves are used in this location, they should be operated from the control room or a remote, accessible location at grade outside the fire area containing the vessel or the train of process vessels protected by the valve. The valve, the electric motor or pneumatic operator, and the portion of supply lines located within the fire area shall be fire-proofed. The remaining supply lines shall be designed and installed to withstand the heat of the fire for 30 minutes.

(b) Electric motor operated or pneumatically operated vapor depressuring valves may be located outside the fire area when the fireproof requirements for valves, operators, and their supply lines inside the fire area are not economical [see Item (a)].

(c) Manually operated vapor depressuring valves, if selected, shall be installed at a safe, accessible location outside the fire area containing the vessel or train of process vessels protected by the valve.

Sizing

In sizing depressuring valves, it should 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 methods given in API RP 521. Vapor depressuring valves may restrict the initial depressuring flowrate to the capacity of the closed pressure relief system and flare. Alternatively, they may be programmed to depressure the required load at a uniform flowrate lower than the capacity of the closed pressure relief system and flare throughout the required time period. The second procedure would prevent overloading the closed pressure relief system and flare.

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.

Depressuring valves are not intended to meet the requirements of the ASME Boiler and Pressure Vessel Code. These valves are used to provide protection in cases where a vessel wall might rupture because of excessive heat from fire or exothermic reactions before the pressure relief valve set pressure is reached.

Disposal of Discharge

Vapor depressuring valves shall discharge into a closed pressure relief flare system. This system is preferred because fluid from depressuring valves is likely to be condensable at the end of a sustained "blow."

Other Considerations

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.

Pressure Relief Valves

The pressure profile in the closed pressure relief system should be examined under depressuring loads to ensure that the outlet flanges of pressure relief valves are not overpressured.

Pressure Vessels

Pressure vessels of copper, aluminum, or other materials that are not sufficiently fire resistant should have protection against fire exposure (for example, external insulation), in addition to a vapor depressuring system