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PIA’s Executive Officer for Standards Ken Kugler outlines the latest developments in AS 2941, Fixed fire protection installations – Pumpset systems.

The latest editions of AS 2941 have effectively banned the application of pressure relief valves (known in other industries as pressure sustaining valves) in fire protection pump applications. There are certain exceptions to this requirement, but overall it does appear to be a confused belt and braces approach.

But first, let’s not confuse the requirements in a fire pump system for a circulation relief valve (or closed head valve as it is known in EN standards) and the pressure relief valve. These two valves (specified in AS 2941 section 3.7) provide totally separate functions despite the fact that they may operate in a similar way.

Circulation relief valves

All centrifugal pumps, especially automatically started fire protection pumps, whether electrically or diesel driven, require protection against damage if operated under a closed discharge valve condition. This may occur due to a false start signal or occur in system testing when the fire water demand is actually zero. A circulation relief valve is required to bypass a small amount of water to prevent the catastrophic pump damage that results from the pump overheating and the water boiling.

It should also be understood that any centrifugal pump operating at very small flows, say less than 10 per cent of its best efficiency point, is operating under a stressed condition. Not only does the water within the pump heat up, but bearing loads are high due to increased radial load, recirculation within the pump and, possibly, increased NPSHR. Operation under this condition will not be tolerated by, for instance, a process pump which is constantly in use because wear is substantially increased and minimum flows are usually restricted to about 30 per cent of best efficiency.

Pressure relief valves

A pressure relief valve (PRV) is required to protect a fire system from over-pressurisation caused by the pump operation. So why did the committee presiding over the standard consider the deletion of pressure relief valves necessary?

The committee’s decision appears to be based mainly on the assumption that if a PRV fails while fully open it will prevent any water entering the protection system – a total disaster. PRV suppliers might argue otherwise, but often installed PRVs are found to have incorrect pressure settings; either too high (providing insufficient water) or too low (providing insufficient pressure) which may actually cause overloading of the pump drivers. Thus, the secondary consideration is obviously to remove the maintenance issue by deleting the PRV altogether and substituting it with an improved initial pump and piping system design.

A third consideration is how to size the PRV. For a fixed speed pump this is relatively easy, but for an over-revving diesel pump the size of the PRV may need to be increased to allow for the additional flow that needs to be bypassed. This requirement for correctly sizing PRVs for diesel driven pumps is not so easily understood.

The committee addressed this by specifying an automatic overspeed shutdown control for the diesel engine.

If a pump’s suction is supplied by a storage reservoir and it’s electrically driven at a fixed speed, for example 2,950rpm, then good engineering design of the piping system should ensure the pump never over-pressurises the system under any flow condition. A PRV is thus not required – it has no job to do.

However, should the pump suction be provided by a town main, then the pump discharge pressure varies in accordance with varying town main’s pressure. Again, good engineering piping design practice should be able to ensure the system piping is designed to an adequate pressure rating to accommodate the variations.

A fire pump driven by a diesel engine is initially locked to a fixed speed at the testing and commissioning of the system. However, the engine itself is a variable speed driver and the speed could inadvertently be increased, say during maintenance (mechanics always like to rev an engine) and the discharge pressure could increase to the square of the speed. As mentioned earlier, this pressure issue is addressed by specifying automatic overspeed shut down of the engine, thus ensuring overspeed and over pressurisation never occur.

This is the theory, but what appears to happen too often is that the designed fire water pressure demand is based on the minimum guaranteed pressure for the town main, while the daily maximum pressure in the main is some hundreds of kPa greater than that nominated. It is often said that “water boards” are unable or reluctant to provide the anticipated maximum town pressure at a particular location, or that where it is nominated it is found to actually be higher in practice.

The AS 2941 method to address applications with highly variable suction pressures is to provide pumps with variable speed drives (VSD). Effectively, it would appear that when PRV’s went out, VSD’s came in. Keeping the pump discharge pressure constant under varying suction pressures is obviously helpful to the fire engineer, enabling pressure to be controlled more precisely, especially in multi-story buildings.

These issues are addressed by the use of VSDs and are specified in the NFPA standards (in the US), and are now included in AS 2941 in Australia. However, both these standards require a fire pump to have a PRV as back-up if driven by electric or diesel VSD drivers. There appears to be doubt about the reliability of VSDs – if the VSD is to fail then the driver is required to operate at the normal design speed (based on the advised minimum suction pressure) causing the pump to over-pressurise the system – thus the requirement for a PRV.

Whatever happened to the kiss (keep it simple stupid) principle to avoid over complication? There is a solution to avoid VSDs and PRVs altogether for pumps operating under varying suction conditions from town mains.

An expensive solution is the installation of a relatively large suction storage tank of required capacity to provide the running time as specified by the sprinkler and/or hydrants codes. But a smaller ‘break tank’ between the town main water supply and the pump set can also solve the problem. The break tank accepts a flow of water from the main under varying suction pressures and the pump operates under a constant suction pressure.

Unfortunately AS 2941 does not allude to the break tank or even note the possibility of its usefulness for these applications. However, section 8 of the fire pump tank standard AS2304 specifies the requirements for these very useful break tanks.

Pumpset driver type

Circulation-relief valve

Pressure-relief valve

Electric

Always required

Not required

Electric with variable speed drive

Always required

Always required

Compression-ignition – air/radiator cooled

Always required

Not required

Compression-ignition – heat exchanger cooled

When required by Clause 3.7.4.1

Not required

Compression-ignition – air/radiator cooled with variable speed drive

Always required

Always required

Compression-ignition – heat exchanger cooled with variable speed drive

When required by Clause 3.7.4.1

Always required

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