07-22-00 22:31
No 31803
Publication :Date :October, 1999 Copyright : Copyright 1999 Chemical Week Publishing, LLC \Volume :106 Issue :11 Page :193
Section : OPERATIONS & MAINTENANCE
Improve the Fire Protection of Pressure Vessels
During a plant fire, a relief valve may not open in time. Designing for this contingency can save a vessel from destruction
Wing Y. Wong UOP LLC
Edited by Peter Silverberg
Pressure relief valves (PRVs) are a favored protection against explosions of pressure vessels due to external fires. Because two Recommended Practices (RPs) issued by the American Petroleum Institute (API; Washington, D.C.) endorse PRVs, many engineers do not realize that PRVs are only one component of a total defense strategy. This article details alternative approaches to protect vessels in addition to using a PRV.
The first step towards understanding starts with a look at the process fluid inside a closed vessel. It can be gas, liquid plus gas, or liquid. Protection techniques depend on the fluid volume and its thermal properties at the first instant of fire. This article will discuss gas-filled and quasi-gas-filled vessels. We will outline the problem and then solutions.
PRVs for gas-filled vessels
A PRV on a gas-filled vessel, exposed to an external fire very likely will not protect the vessel -- it will just provide a false sense of security. A flame has hot spots and cool spots. The heat-transfer coefficient between the wall and the gas in the vessel is very small. The steel wall cannot cool down. Tests and actual fire incidents have confirmed that when a gas-filled vessel is exposed to a fire, the vessel wall develops overheat spots where the flame is hottest. The vessel will fail due to heat stress on the wall long before its internal gas pressure reaches the set pressure of the PRV.
Section 3.15.1.2 of API RP 521 [1] warns design engineers about the potential consequences of such a thermal failure. API RP 521 does indicate that a PRV alone may not be adequate to protect gas-filled vessels. At the same time, API RP 520 [2] provides detailed fire-load calculation procedures for gas-filled vessels. Most petroleum refiners use these equations to size PRVs. However, a gas-filled vessel cannot be protected by a PRV alone.
Equations 1 and 2 are used to size PRVs. They are based on the following assumptions:
-- The vessel is not insulated
-- The vessel wall temperature (recommended maximum: 1,100 degrees F) will not weaken the wall below its rated rupture stress.
(1)
(This equation is not available electronically. Please see the October, 1999 issue.)
(2)
(This equation is not available electronically. Please see the October, 1999 issue.)
These assumptions are the source of the problem. The biggest shortcoming of using a PRV is a product of real-life materials. Most market-available PRVs will fail below 1,100 degrees F. The highest temperature capability of an available steam valve from valve manufacturers is about 1,050 degrees F. Most other valves with the best available elastomer or plastic seals (such as stainless-steel-filled Teflon and Kalrez) do not work above 550 degrees F.
Some carbon steel vessels fail below 1,100 degrees F. In refinery fires, the flame temperature can exceed 1,400 degrees F. API RP 520 indicates that an unwetted steel vessel with ASTM A515 Grade 70 steel will rupture in about 2.5 minutes at 1,300 degrees F [3]. Figure 1 shows an example of the interaction of time and temperature at 1 atm.
Effective fire protection
There are other measures that can be taken to effectively protect gas-filled vessels against external fire:
1. Proper location of the vessel. This is key in planning for protection against external fire. Refineries and chemical plants should seriously consider the measures below, especially for vessels containing H2, or other explosive or flammable gases:
(a) Isolating the high-risk, gas-filled vessels by surrounding them with a firewall and moving them away from the main process area.
(b) Burying the gas-filled vessels underground and covering them with earth
(c) Raising the gas-filled vessel to an elevation 25 ft above grade. Fire reports confirm that flame height usually stays below 25 ft.
2. Install external fireproof insulation on a gas-filled vessel. Insulation can reduce the heat absorption from a fire to 30% or less of that of a non-insulated vessel. However, design engineers must be aware that:
(a) The insulation materials must function effectively while withstanding flame temperatures up to 1,660 degrees F during a fire.
(b) The insulation should withstand fire exposure for two h.
(c) Insulation should resist dislodgment by high-pressure water streams used for fire fighting. Therefore, stainless steel jacketing and bonding to retain the insulation shape is required.
3. Design an automatic vapor-depressurizing system [1] Vapor depressurizing is one of the most-effective emergency approaches for gas-filled vessels exposed to an external fire. Such a system can greatly reduce the severity of the fire, with subsequent consequences of vessel failure, including high-pressure gas erupting from the weakest overheated spot of a vessel. Consideration should be given to installing an automatic vapor depressurizing system on all major gas-filled vessels with operating pressure above 50 psig.
4. Equip a reliable fire-monitoring system and a rapid-response fire fighting team.
5. Install a well-designed water deluge system (WDS). This should effectively allow very little or no heat input to reach a vessel. A carefully designed WDS can effectively lengthen the time of exposure to the external fire while limiting serious damage to the insulation and vessel wall.
However, API Recommended Practices do not incorporate a WDS in the PRV calculation, because most of these systems would either not function properly or be destroyed during a fire. A WDS is a good measure to protect a vessel against a small fire, but if it is not well-maintained, it will be unreliable. It will not resist a large fire. Design engineers should read the NFPA (National Fire Protection Assn.; Norwood, Mass.) Handbook to learn the basic requirements for an effective WDS.
High-boiling point liquid
Engineers should also take extra precautions when dealing with a high-boiling-point liquid. The design temperature must be above the fluid's boiling point at its relieving pressure. If it is difficult to build a vessel of suitable metal, then apply the measures for protecting gas-filled vessels.
The calculation of the fire-relieving capacity for the high-boiling-point liquid is different from the rest of this article. There are two scenarios for a vessel with high-boiling point liquid:
If the vessel is not full, the situation is very similar to the gas-filled vessel because there is no or very little vapor generation at the relieving pressure. The same sort of hot spots show up on the walls, while the contents stay cool.
If the vessel is full, the required fire-relieving capacity is equal to the thermal relieving capacity, since there is very little or no vapor generation after its initial thermal expansion. Engineers can refer to API 520 and Reference 4 for sizing the required thermal relief valves.
In both cases, avoid connecting the PRV discharge line to the common flare header. It is better to use a separate discharge line. The problem is that high-boiling-point liquid might crack during a fire, form coke and plug the line. If the vessel has no vent, it will probably break apart. As with gases, the most conservative approach is to isolate or bury such vessels.
Be aware that some viscous high-boiling-point liquids may need hours to boil. The equations say that no PRV is required; nevertheless, hot spots on the vessel wall can still develop, and eventually rupture, before fire-fighting teams arrive. Good fire protection approaches are still needed.
Low-boiling-point liquids inside a vessel will boil off into the vapor phase when the vessel is heated. There is a good heat-transfer coefficient associated with this phase change. It buys time before the vessel becomes a gas-filled vessel. It will be the designer's task to figure out if the firefighting team can arrive before the vessel wall fails. The amount and properties of this liquid determine the time.
A boiling liquid buys time
When a fire is catastrophic or uncontrolled, even a WDS can be melted down. Basically, everything in the fire zone could be wiped out. But, for vessels with large liquid inventory (``wetted-surface''), there is usually more time to hold the wall temperature below dangerous levels.
For an external fire, the only function of a pressure relief valve is to limit the damage by having the PRV open prior to vessel wall rupture. Don't count on a PRV being good protection in a large fire, even if the vessel has a large inventory of liquid.
When a vessel with a low liquid inventory is exposed to a fire, the enormous amount of heat absorption from the fire could quickly deplete the liquid inventory by evaporation. After that short period of time, the vessel becomes a gas-filled vessel. The installed PRVs can not protect low-liquid inventory vessels from external fire any better than they can protect gas-filled vessels.
The first design task is to classify the category of low inventory vessels as either belonging to wetted-surface vessels or gas-filled vessels. In other words, what is the boundary separating small and large inventories?
A wetted-surface vessel will have enough liquid inventory that it will undergo the vaporization process until the firefighters arrive. If the liquid inventory will be depleted in less than 15 -20 min, treat the low-liquid-inventory vessel as a gas-filled vessel.
The actual unprotected time should be defined by design engineers based on the average response time from the fire fighting team, the location of available fire fighting equipment, and the fire protection conditions of the plant. The low limit of the total response time should not be less than 15 min.
Calculation method
Let us set up calculations for the time required for vaporizing all the liquid inventory in a vessel. This makes it possible to test the time and the setting of the PRV. The following calculations assume that the liquid level is below the 25 feet of effective fire height specified by Reference 3.
Step 1 -- Calculate the actual liquid volume in a vessel. For simplicity, assume the partial volume of liquid in a vessel is proportional to the ratio of wetted surface area to total area. If a vessel has a boot, its volume should be added.
(3)
(This equation is not available electronically. Please see the October, 1999 issue.)
Step 2 -- Calculate the time required for vaporizing all the liquid in a vessel. First, find the time required to heat the liquid to the boiling point:
(4)
(This equation is not available electronically. Please see the October, 1999 issue.)
Calculate the time required to vaporize the liquid inventory:
(5)
(This equation is not available electronically. Please see the October, 1999 issue.)
Step 3 -- Obtain the total time required to deplete the liquid inventory:
(6)
(This equation is not available electronically. Please see the October, 1999 issue.)
If the calculated time is less than 15 min, then treat it as a gas-filled vessel. The PRVs on the vessel can not prevent a vessel-wall meltdown.
If the calculated time is more than 15 to 20 minutes, we can feel confident using the wetted-surface-area procedures. Size the PRV with Equations 1 and 2. Fine-tuning the calculation, using Equation 6, may add credit for the metal wall. Figure 2 can be used to estimate that.
Step 4 -- Determine if the vessel pressure could reach its set pressure. In certain cases, after the last drop of the liquid is vaporized, the vessel pressure may reach its set pressure and the PRV on the vessel will lift. It is especially true if the set pressure of the vessel is low. Therefore, in addition to the above time criterion, design engineers should also perform a rough calculation to determine the final pressure of the vessel with the last drop of liquid.
Assume: (1) The temperature inside vessel has reached the boiling point
(2) The liquid and vapor inside the vessel is a single-component hydrocarbon, or other pure material
(3) The vapor is an ideal gas
For the initial vapor phase, the vapor moles can be calculated as:
(7)
(This equation is not available electronically. Please see the October, 1999 issue.)
The liquid moles can also be calculated by:
(8)
(This equation is not available electronically. Please see the October, 1999 issue.)
The total moles in the vessel is:
(9)
(This equation is not available electronically. Please see the October, 1999 issue.)
The final pressure of the vessel, when all the liquid is vaporized, is:
(10)
(This equation is not available electronically. Please see the October, 1999 issue.)
A final confirmation: Check this against the PRV set point.
Wing Y. Wong is a senior specialist in pressure relieving systems for UOP LLC (25 East Algonquin Road, Des Plaines, IL 60017; Phone: 847-375-7376; Fax: 847-391-2758; E-mail: wywong@uop .com). Mr. Wong has written many articles on presure-relieving systems. He has an M.S. in chemical engineering from the University of Alberta and a B.S. from East China Instutite of Chemical Technology. He is a registered P.E. in Illinois.
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