Fire Safety Journal, 14 (1989) 303 - 304 303

Book Review

Liquified Petroleum Gas: Vol. 1, Large Bulk Pressure Storage and Refrigerated LPG. Model Code of Safe Practice, Part 9

a Code jointly prepared by the Institute of Petro- leum, The Institute of Gas Engineers and the Liquified Petroleum Gas Industry Teehnieal Asso- ciation (U.K.), published on behalf of the Institute of Petroleum, London, by John Wiley & Sons, Chiehester, February 1987, 89 pp.; price £38.50; ISBN 0471 916129.

This code of practice is a much revised and expanded version of an edition published in 1967. Industrial LPG installations (smaller scale and domestic equipment will be dealt with in a subsequent volume) have multiplied many times both in scale and number since that date. There have also been numbers of extremely destructive failures of which the worst was that at the Pemex installation near Mexico City in which 500 people lost their lives.

What is the purpose of a code of practice ? It seems, essentially, to be a resource with which the less experienced or more junior engineer can be equipped before being sent away to perform a piece of routine design work. LPG installations are common enough for their design to be routine but the conse- quences of failure are horrific enough to be very far from routine. It follows that a code of practice should be absolutely right, easy to use and clearly written.

How does this code of practice measure up to the above criteria? The section on "hazards" is enlarged but not conspicuously more useful than the 1967 version. For ex- ample, the anaesthetic properties of LPG are mentioned without any instruction as to the concentration at which they begin to appear --a vital piece of information when issuing permits to enter vessels. The vapour pressure of commercial butane is quoted as 0 bara (not barg) at 0 °C. This doesn' t inspire confidence in the accuracy of the other numerical data.

It is s t a t e d - c o r r e c t l y - that when water is present in LPG icing may be a problem. The rapid cooling of LPG by flashing when the pressure is reduced is also mentioned but any

connection between these two hazards is not spelled out.

The 1967 version wisely tabulated a series of minimum separation distances between LPG storage vessels. The modern approach is to specify values of the minimum allowable thermal flux from a fire at another vessel for unprotected and protected (insulation, water irrigation or both) vessels. The recommended methods for calculating this radiant flux depend on the sequential use of a series of essentially empirical equations which depend in their turn on a series of assumptions about the size and shape of the original fire, wind speed, etc.

The correlations for thermal output at source of pool LPG fires at various elevations and inclinations seem reasonable and are easy to use. The same can be said for the nomo- grams for view factors, atmospheric transmit- tance, etc.

Unfortunately, this cannot be said for Ap- pendix 2, which provides methods for calcu- lating gas velocity, temperature and jet dimen- sions, e.g., for fires at relief valve outlets. At the very least, it is confusingly written, not all the symbols used are defined, the units are inconsistent (psi, °K, kg, but mols, etc.) and the thermodynamic basis of the material is suspect.

Since in most relief valve discharges for LPG the pressure ratio is far less than 0.5, flow through relief valves and relief valve headers is usually assumed to be choked. This means that m = me. It is certainly difficult to imagine cases in which m > me, but by the wording of Appendix 2 it seems that the author does !

Even were all the stages of the calculation unimpeachable, there are many of them and the cumulative error may well be consider- able. The simple use of a minimum separation distance, as in 1967, would in my opinion be preferable. (To be fair, such a minimum sepa- ration is given as a "fail-safe" figure.)

Some important questions are not clearly answered. For example, bunding is not recom- mended for pressure LPG storage but by im- plication it is for refrigerated storage.

C) Elsevier Sequoia/Printed in The Netherlands

304

Although the refrigerated vessel is less hazardous because it is not pressurized and has the protect ion afforded by its insulation, the prospect of a pool fire in the bund engulfing it is surely most unwelcome.

Protect ion of vessel surfaces by water irri- gation will work only if all the affected sur- face is wet. "Armpi t " areas such as those be- hind and under vessel supports are particular- ly liable to remain dry and special at tention must be paid to them, a priority nowhere mentioned here.

Disappointingly, nonferrous (copper or brass) valves and small-bore tubing are still permit ted without any mention of a fire-safe steel shut-off valve installed between any such section and the vessel.

Great at tention is paid to eliminating any chance of combust ible vapour clouds reaching the site boundary. Why? If sources of igni- tion are the worry, ample chance of this is given by electrostatic effects in discharge jets or sparks from hot or even cold work. Conver- sely, at Pemex the fires beyond the fence

were caused by radiation from BLEVES (boiling-liquid expanding-vapour explosions) or burning LPG broadcast by tanks and sec- tions of tanks converted to rockets or projec- tiles after their failure. A more wor thy design aim might be to reduce the chance of emis- sions in the first place.

Finally, there is a profusion of anodyne phrases of which my favourite is "an adequate level of safety for plant operators and the gen- eral public." What on earth is this supposed to mean? In presenting a safety case to HSE (Health and Safety Executive) [or CIMAHH (Construction Industry Manufacturers Asso- ciation)], one would be required to quantify such risks. There is no reason why IP should be mealy-mouthed. This document is indeed a useful reference for the designer of industrial LPG installations, but it can be no substitute for the close at tention of highly experienced engineers with a vivid appreciation of the con- sequences of failure of their designs.

D. H. GLASS