2000 Hook-up Book

Testing Steam Traps

Table 16: Steam Trap Discharge Modes Mode of Operation

SYSTEM DESIGN

Full or

Usual

Trap Type

No Load

Light Load

Normal Load

Overload

Failure Mode

Float &

Usually continuous but may

Closed,

Thermosatic

No Action

cycle at high pressure

Continuous Continuous

A.V. Open

Inverted Bucket

Small Dribble

Intermittent

Intermittent

Open

Balanced Pressure Thermostatic

No Action

May Dribble

Intermittent May blast at high pressures

Continuous

Variable

Bimetallic

Usually Dribble

Thermostatic

No Action

Action

Continuous

Open

Usually continuous with blast at high loads

Impulse

Small Dribble

Continuous

Open

Disc Thermo-Dynamic

No Action

Intermittent

Intermittent

Continuous

Open

What must be done, using all audible and visual clues, is to detect normal or abnormal cycling of the discharge. Even this method is very fallible, since the mode of operation of different trap types if not nearly so well defined as is sometimes thought. Table 16 lists some of the possi bilities and allows the problem to be seen more clearly. It is seen that the “signal” to be obtained from the trap, whether visual, audio or temperature, is usually going to be so ambiguous as to rely largely on optimism for interpretation. The one trap which is fairly positive in its action is the disc thermodynamic type—if this is heard or seen to cycle up to ten times per minute, it is operating normally. The cycling rate increas es when the trap becomes worn and the characteristic “machine gun” sound clearly indicates the need for remedial action. Spira-tec Leak Detector System Logic says that if it is not possible to have a universally applicable method of checking steam traps by examining the traps them selves, then we must see if it can be done by checking elsewhere. This is what Spirax Sarco has done with the Spira-tec system. See Fig. 61 (page 58). The Spira-tec detector cham ber is fitted into the condensate

line on the inlet side of the trap. If there is, at this point, a normal flow of condensate towards the trap, together with a small amount of air and the steam needed to make up heat loss from the body of the steam trap, then all is normal. On the other hand, an increased flow of gas along the pipe indicates that the trap is leaking. The chamber contains an inverted weir. Condensate flows under this weir and a small hole at the top equalizes the pressure on each side when the steam trap is working normally. An electrode on the upstream side of the baffle detects the presence of conden sate by its conductivity which is much higher than that of steam. By plugging in the portable indi cator, it is possible to check if the electrical circuit is complete when a visual signal indicates that the trap is working. If the trap begins to leak steam, then the pressure on the downstream side of the weir begins to fall. The higher pres sure on the upstream side drops the condensate level below the electrode and exposes it to steam. The “conductivity” circuit is broken and the indicator light gives a “fail” signal. The advantage of the system lies in the very positive signal which does not require experi ence of personal judgement before it can be interpreted.

Using suitable wiring, the test point can be located remote from the sensor chamber or it can have a multi switch to allow up to twelve (12) chambers to be checked from a single test loca tion. When appropriate, an electronic continuous 16-way checking instrument can monitor the chambers and this is readily connected into a central Energy Management System. The object of detecting leak ing steam traps is to correct the problem. This can mean replace ment of the whole trap, or perhaps of the faulty part of the internal mechanism. It is very useful indeed to be able to check a repaired trap in the workshop before it is installed in the line, and many repair shops now use a Spira-tec chamber as part of a bench test rig. The diagram shows a simple hookup which allows suspect or repaired traps to be positively checked. (Fig. 60) Cost Of Steam Leaks The installation and use of the Spira-tec units does involve some cost, and it is necessary to com pare this with the cost of steam leakages to see if the expenditure is economically justifiable. Since all equipment must wear and eventually fail, we need first an estimate of the average life of a steam trap. Let us assume that in a particular installation, this is,

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