Reciprocating Compressors - Protection against Crank Case Explosions

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-5705

Reciprocating Compressors Protection against Crank Case
Explosions
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Engineering Design Guide:

Reciprocating
Compressors - Protection
against Crank Case
Explosions

CONTENTS

SECTION

1

SCOPE

1

2

OIL MIST/AIR MIXTURE EXPLOSIONS

2

3

PREVENTION AND PROTECTION

3

3.1

Design

3.2

Maintenance and Operation

FIGURES

1

FLAMMABILITY LIMITS AND SPONTANEOUS IGNITION REGION
FOR MIXTURES OF LUBRICATING OIL VAPOR IN AIR.


1

SCOPE

This Engineering Design Guide describes the precautions that must be taken to
prevent crankcase explosions in reciprocating compressors and to provide
protection against them.

2

OIL MIST/AIR MIXTURE EXPLOSIONS

Spontaneous ignition and explosion of oil mist/air mixtures can occur when the oil
by weight in the mixture is between approx. 4% and 17% and the temperature
above between 230°C. and 420°C depending on the oil content (see Fig. 1). The
maximum pressure generated by an oil mist/air explosion is between 7 and 8 bar
gauge. In practice, these conditions can arise from local hot spots in a
compressor crankcase due to say, tight little end bush clearances. If a hot spot
occurs in a crankcase it is probable that the oil content of the atmosphere in the
crankcase will rapidly exceed 17% by weight. This means that there is a high
probability of an explosion occurring if a cover is removed and air admitted
before the local high temperature has been dissipated.

3

PREVENTION AND PROTECTION

3.1

Design

At the design stage the only method of ensuring prevention of crankcase
explosions is to provide for a continuous purge of an inert gas such as nitrogen to
the crankcase. The gas must however be clean and dry and this is often difficult
to provide, expensive and sensitive to mal-operation. It is considered that a more
practicable solution is to concentrate on protection on the assumption that an
explosion may occur at some time.


3.1.1 Crankcase Pressure
Two methods of obtaining reliable protection at reasonable costs are available:
(a)

The crankcase must be designed to withstand safely an internal pressure
of 8 bar gauge or:

(b)

Pressure relief valves must be fitted to the crankcase. Suitable valves are
available at reasonable cost. In this case it is necessary to ensure that
adequate relief area is incorporated, based on the following values:

(I)

Recommended:
70 mm2 of relief area per liter of crankcase volume (i.e. gas space in the
crankcase).

(2)

Minimum acceptable:
50% of recommended value.

3.1.2 For either of the arrangements in Clause 3.1.la or Clause 3.1.lb it is
essential that no vent cover or attachment to the crankcase is loosely
fitted; they must in all cases be securely attached. Vents should be fitted
with wire gauze filters which will also be effective as flame traps.

3.2

Maintenance and Operation

The dangers of crankcase explosions can be further reduced if the following
points are taken into consideration by maintenance and operating staff:
(a)

Particularly careful attention should be paid to running clearances during
maintenance.

(b)

A short initial run after any major maintenance should be followed by a
check for signs of local high temperatures. It is of particular advantage to
purge the crankcase with a clean, dry inert gas during initial running if this
is available.


(c)

Covers should not be removed from a hot machine until it has
had time to cool down.

(d)

Covers should never be temporarily fitted with two or three nuts only - they
should be fully and securely fastened down. This is particularly important
where the basis protection relied on is the ability of the crankcase to
withstand the full possible internal pressure of 8 bar gauge. Temporary
fitting of a cover with only a few nuts will in this case completely remove
the intended protection.


FIGURE 1 - FLAMMABILITY LIMITS AND SPONTANEOUS IGNITION
REGION FOR MIXTURES OF LUBRICATING OIL VAPOR IN AIR


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