fm spacing of facilities in outdoor chemical plants

Factory Mutual 7-44 Property Loss Prevention Data Sheets 17-3

September 1975 Revised December 1998

of 6

SPACING OF FACILITIES IN OUTDOOR CHEMICAL PLANTS

Table of Contents Page

FLAMMABLE VAPOR OR GAS EXPLOSIONS IN THE OPEN .................................................................. 2 FLAMMABLE VAPOR OR GAS EXPLOSIONS IN PROCESS EQUIPMENT ............................................ 2 OTHER VESSEL FAILURES ........................................................................................................................ 2 DETONATIONS ............................................................................................................................................. 3 DAMAGE FROM BLAST WAVES ................................................................................................................ 3 SPACING OF FACILITIES ............................................................................................................................ 4 SEPARATION GUIDELINES ......................................................................................................................... 5

List of Figures Fig. 1A. Spacing distances for outdoor chemical processing equipment. (English units) ............................ 3 Fig. 1 B. Spacing distances for outdoor chemical processing equipment. (Metric units) .............................. 4

FACTORY MUTUAL IB~~® ©1975 Factory Mutual Engineering Corp. All rights reserved. No part 01 this document may be reproducqd, stored in a retrieval system, or transmitted, in Whole or in part, in any form or by any means. electronic. mechanical, photocopying, recording, or otherwise, without written permission of Factory Mutual Engineering Corp.

7-44 17-3 Chemical Plants Factory Mutual Property Loss Prevention Data Sheets

The extent of a single fire or explosion loss in an outdoor chemical plant can be minimized by providing separation between process units, storage facilities, and important buildings. The spacing needed is based on many factors, including:

1. The severity and probability of potential fires or explosions.

2. The relative value and importance of facilities.

3. The susceptibility of building components and outdoor equipment to direct damage.

4. The susceptability of building contents and equipment contents to direct and subsequent damage.

Where the only hazard to be considered is fire, Factory Mutual recommended practices for flammable liquid storage tanks, exposure from buildings, or exposure from outdoor storage of other commodities give distances which will furnish acceptably low levels of damage from heat and prevent fire spread due to the applicable fire exposure.

The different types of explosions which may be expected in chemical plants may be classified as vapor or gas explosions outside of equipment, vapor or gas explosions in process equipment, other vessel failures, and detonations.

FLAMMABLE VAPOR OR GAS EXPLOSIONS IN THE OPEN

Flammable vapor or gas explosions are normally deflagrations. A deflagration is a propagating reaction where the energy transfer from the reaction zone to the unreacted zone is accomplished through ordinary heat or mass transfer. The reaction progresses at a rate below sonic velocity. Where a small explosion takes place in the open, the energy generated in the form of compressed gases is dissipated by expansion as fast as it is generated, and the primary effect on the surroundings is intense heat. If the vapors enter an enclosure before ignition and the hot gases are confined, a pressure buildup can result until vented by failure of the enclosure. There have been a few cases where massive vapor or gas release has covered many acres and where the reaction increased in velocity to the point where a damaging shock wave was produced.

There were 108 known accidental unconfined vapor-cloud explosions in a recent 42-year period. For additional details see Data Sheet 7-42.

FLAMMABLE VAPOR OR GAS EXPLOSIONS IN PROCESS EQUIPMENT

Where the vapor space of a process vessel is in the explosive range, a deflagration reaction can occur. The hot gases are confined so pressure can build up to about six to ten times the original absolute pressure, If this pressure is above the failure pressure of the vessel, the vessel may fail catastrophically. The confined energy is then released all at once, A shock wave similar to that produced by a detonation can result. The shock wave moves at sonic velocity and does damage quite similar to that done by detonation of TNT or other explosive, Missiles also are produced by pieces of the vessel. These are usually larger than those produced by a detonation and do not travel as far.

After the vessel failure, the deflagration reaction may continue, but it is now vented. Since the deflagration wave is slower than the shock wave and behind it, it usually does not add to the explosion damage.

OTHER VESSEL FAILURES

When an exothermic reaction occurs in a vessel without sufficient cooling or venting, the reaction may accelerate, generating pressure from boiling vapors or decomposition products, until the vessel reaches its failure pressure, The energy release then is similar to that from a confined vapor explosion, except that the liquid is usually above its normal boiling point. When the pressure is released by vessel failure, a large proportion of the liquid instantaneously vaporizes, adding to the shock wave resulting from the expansion of the confined gases already present. Unless a chemical subject to detonation is present, further chemical reaction or combustion of gases does not add to the shock wave.

©1975 Factory Mutual Engineering Corp. Ail rights reserved,

7-44 Chemical Plants 17-3 Factory Mutual Property Loss Prevention Data Sheets

DETONATIONS

A detonation is a propagating reaction which progresses at or above sonic velocity in the reactant. The shock wave produced is somewhat similar to that produced by commercial explosives or by catastrophic rupture of a vessel. If the energy can be predicted in terms of TNT equivalent, the extent of damage to structures or equipment of varying types of construction can be predicted with reasonable accuracy.

Sometimes detonable materials such as acetylides or organic peroxides are accidently produced or accumulated. If these can be anticipated and predicted, it may be possible to provide safe isolation for such hazardous processes.

DAMAGE FROM BLAST WAVES

Studies indicate that the characteristics of the blast waves from vessel ruptures and from detonations are not identical. However, they are so similar that a common set of curves can be derived which agrees reasonably well with theoretical calculations, experimental tests, and loss experience. These curves are Fig. 1A (for English units), and Fig. 1 B (for metric units).

'"3 100 ::l., d:

VOlume d Veual

Fig. 1A. Spacing distances for outdoor chemical processing equipment. (English units)

USing the curves, the energy released by expanding gas liberated by catastrophic rupture of a vessel can be approximately related to the equivalent weight of TNT. This is done by using the volume of the vessel and the failure pressure of the vessel. As an estimate the vessel failure pressure may be taken as four times the design pressure. If a deflagration is confined to the vapor phase and the liquid in the vessel is below its atmospheric boiling point, use only the volume of the vapor phase under the worst credible condition. If the liquid is heated more than 50°F (28Q C) above its atmospheric boiling point so that a substantial amount of the liquid will turn to vapor on vessel failure, the failure pressure should be increased by a percentage equal to the percent of liquid in the vessel, and the full vessel volume should be used.

For example, a 2,000-galion (7.5-rn3 ) reactor contains 1,000 gallons (3.8 m3) of liquid and is subject to a ther

mal explosion with failure expected at 150 psig (1000 kPa) (10 bars). Since the reactor will be 50% full of an overheated liquid, the effective pressure at failure will be 150 psig x 150% 225 psig (1500 kPa) (15 bars). With a reactor volume of 2,000 gallons (7.5 m3

) from Fig. 1, the TNT equivalent will be about 15 Ib (6 kg).

©1975 Factory Mutual Engineering Corp. All rights reserved.


IOO~~--~~~~--~~~~~~~~~~~~~~~~~~~~~~~~~~__ (I) .I 10 Cu, Mele"

Volume of Ves!el

Fig. 18. Spacing distances for outdoor chemical processing equipment. (Metric units)

Once the TNT equivalent is obtained, it is possible to estimate the approximate radius of severe damage using the same curves. Severe blast damage normally occurs over a greater distance than missile damage. Chemical process equipment can normally withstand substantial blast pressures but is more subject to missile damage. Spacing in accordance with that for light wall sheathing is recommended to minimize missile damage.

In the above example where a yield of 15 Ib (6 kg) of TNT is expected, a spacing of about 110ft (33 m) is specified for light sheathing and framing, load-bearing masonry construction, and process vessels. Extensive damage to large windows may be expected for about 220 ft (66 m). Such window damage may be acceptable unless vital facilities inside such as computer or control equipment could be damaged by glass fragments or the blast wave entering the room. Beyond 220 ft (66 m) some windows may be broken, but interior damage should be light. Wherever there is an explosion hazard, damage from missiles, vapor cloud explosions, and ensuing fires is expected within 100 ft (30 m). Greater spaCing for fire exposure only may be needed as outlined in applicable Factory Mutual recommended practices.

Some materials used in chemical processes may be subject to detonation. These include some organic peroxides, acetylenic compounds, commercial explosives, and compounds of similar instability. If the quantity and type of materia'l present is known and the TNT equivalent can be obtained, Fig. 1 A or Fig. 1 B may be used to determine the extent of expected damage.

SPACING OF FACILITIES

Separation or other protection is normally provided between an exposed facility which must be protected and a hazard which may constitute a fire or blast exposure, or both. Many facilities expose one another because they constitute both high values and a severe hazard . . Sufficient spacing between major process blocks, tank farms, important buildings, and major plant utilities is normally needed to confine fire and explosion damage to the major unit of origin.

Equipment and structures within a process block are usually spaced in accordance with production requirements. It is seldom practical to isolate such individual facilities except important control houses and unusually hazardous process equipment.

©1975 FactoI)' Mutual Engineering Corp. All rights reserved.

7-44 Chemical Plants 17-3 Factory Mutual Property Loss Prevention Data Sheets

Separation or other physical protection is normally needed to protect the following from an exposing fire or explosion:

1. Process equipment of high value or importance to production, within buildings or outside.

2. High value storage facilities, such as large tanks, tank farms, warehouses, or yard storage.

3. Utility buildings and equipment vital to production or fire protection, such as major boilers, compressors, water and electrical services, pump houses, cooling towers, and maintenance shops.

4. Important control rooms and administrative buildings.

The following can be sources of fire exposure and need separation from high value processes.

1. Large quantities of flammable liquids or gases in a tank, tank farm, process area, processing building, drainage area, flare stacks, or burning pits.

2. Large quantities of ordinary combustibles in a building or in yard storage.

The following can be the source of a flammable gas or vapor cloud, which can explode in the open or enter other buildings: processes or storage facilities at such temperatures and pressures that release of large quantities of flammable vapors or gases can be expected upon failure of equipment.

The following can be sources of blast or missile damage due to rupture of process vessels or detonations, and need separation from high value processes:

1. Processes or storage containing unstable or explosive materials.

2. Process reactions of an exothermic nature where, because of newness, lack of complete investigation, or nature of the reaction itself, a strong possibility of the reaction getting out of control exists. Included would be some nitration or polymerization reactions.

3. Processes using equipment of large volume which must be taken through the explosive range at periodic intervals for maintenance purposes.

4. Processes which have a significant history of explosions.

5. Unprotected process or storage tanks under pressure where weakening of the pressurized shell can result from an external exposure fire.

6. Processes at pressures over 5000 psi (34.5 MPa) (345 bars).

Many facilities will fall into more than one group. Separation needed is the greatest distance, based on the most severe combination.

SEPARATION GUIDELINES

The following guidelines are for fire separation and predictable explosion separation for the planning of new plants, additions to existing plants, or the evaluation of existing plants. Vapor cloud explosions in the open can cause damage in excess of practical separation distances.

Judgement must be used to determine the extent of damage which can be expected, and the separation or other protection needed. It may not be possible to rearrange existing plants to provide the needed spacing, but changes made over the years should progress toward a satisfactory level of safety. Unusually severe hazards should be minimized through extraordinary preventive procedures such as blast resistant barricades, until adequate isolation can be provided.

1. Separation between a high value facility and fire exposure:

Distances for fire exposure only should be based on applicable Factory Mutual recommended practices for exposure protection, flammable liquid storage tanks, yard storage, etc. For flammable liquids, drainage facilities, dikes, and the slope of land should be taken into consideration.

2. Separation between a high value facility and a vapor cloud exposure:



Because of the unpredictibility of the area which may be affected by a release of flammable vapors or gases in the open, a separation distance beyond that required for fire exposure in guideline No.1 or blast exposure in guideline NO.3 is seldom practical. To minimize the possibility of entry of vapors into buildings, the distance should be not less than 100 ft (30 m); more, if ground slopes so that flow is toward the building, or if discharge of vapors is very likely as from flare stacks or normally venting relief valves.

3. Separation between a high value facility and a blast or missile exposure:

a. For materials such as LP gas, ethylene oxide, hydrogen peroxide, organic peroxides, commercial explosives, ammonium nitrate, etc., for which Factory Mutual loss prevention practices are provided, use the specific spacing recommendations contained therein.

b. For other blast or missile hazards, use spacing guide lines in Figures 1 and 2, with a minimum of 100 ft (30 m). Where needed distances cannot be provided, the exposing hazard should be barricaded by blast and missile resistant construction or the exposed facility should be of blast and missile resistant construction. (Design data is available in Structures to Resist the Effects of Accidental Explosions, Departments of the Army, Navy and Air Force Manual (TM 5-1300, NAVFAC P-397, AFM 88-22) November 1990 Edition.)

There is no NFPA standard on this subject.

FMElPC June 1975


fm spacing of facilities in outdoor chemical plants

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