Life Safety Generators The Inclusion Of A Locked Rotor

The following opinion piece on the application of diesel generator systems within a building services environment has been developed following conversations with various consultants, viewing many similar schemes and proposals, and working with a practical knowledge around the subject matter.

Introduction

Up to around 10 years ago a “typical” building requiring a sprinkler system would have been serviced either by a secondary DNO supply or the emergency power for the sprinkler system would have been via a fire pump directly fed by a diesel engine.

The reluctance of DNO’s to provide a secondary supply to buildings, changes in the law relating to disabled access and egress, and the proliferation of high-rise residential properties has fuelled the growth in the requirement for life safety generating sets. Whilst there have been several developments in specification over recent times, the Grenfell Tower fire has prompted the consideration for the focus around sprinkler systems alone. With the introduction of some additional requirements, a typical life safety generator is now often required to provide the alternative supply for emergency plant which includes sprinklers, lifts and extract fans. Building and fire regulations haven’t kept up with events in the building services environment and “real world events”, plus cost pressures have led to inconsistencies in the way in which specifications are developed and building regulations are applied. In the era of the diesel-powered sprinkler pump, as single pumping arrangement was deemed to be sufficient. Now, in the age of risk reduction and risk aversion, a backup pump is a typical requirement. A relatively new innovation or requirement, which has been imported from the US, is the need to be able to start the second pump when the first pump is still in is failed state – that is, locked rotor. This applies equally regardless of if the pump start is on mains or on generator. The consequence of this is a huge increase in the size of the generator required to manage this process.

Locked Rotor

The inclusion of the “locked rotor”, second motor start requirement into specifications, has a major impact on the rating of the generator. The first pump is part way through its startup phase (star) and the rotor locks effectively causing a short circuit current to flow. This immediately requires the second pump to be started, hence the supply must be capable of managing the starting current of both sprinkler pumps at the same time. The life safety generator, when used for example in a residential setting, is an expensive item of plant. Teams prefer not to have such an item on site, meaning the generating set typically ends up either in a basement (with an inadequate allowance of the space required and a very limited provision of ambient air etc), or on the roof where they create noise and can cause vibrational issues on a typical light weight residential structure. Therefore, optimising the size and capacity of the generator key.

Under short circuit conditions, the output voltage of the generator will tend towards zero, however it is a design requirement (BSENISO 8528) that the set should be able to deliver maximum current for 10 seconds. In the world of harmonised standards, this provides sufficient time for the protective device feeding the short circuit to trip and clear the fault. The adoption of sprinkler pumps in a more conventional auto duty / standby arrangement, would allow the fault on the first pump to be cleared by the protective device. We then allow the generating set five seconds to recover to steady state conditions before starting the standby pump. By doing this:

We can reduce the rating of the generating set
An essential and time sensitive vital piece of safety plant is not unnecessarily “stressed”
The second pump is not started under what are worse case conditions – i.e. low voltage at the generator terminals; risk damaging that pump; in the worse case risk stalling the generator and further endangering lift

This however is not permitted under current regulations hence we must proceed with design considerations as suggested below and provided in more detail in other white papers on the WB Power Services Ltd. website.

Specification of the Generating Set

As with any generator application there are basic steps which need to be followed. The aim in all cases is to optimise the rating of the generator by ensuring that the load is managed on in such a way that there is a close as possible parity between the running load current of the set and the starting load current requirements. In a life safety or water treatment applications, (with multiple or large pumps) this isn’t always possible. It is essential that worse case starting conditions are identified, starting sequence developed, operating parameters of the load equipment quantified and the generator rating and scope of performance of the generator developed from that. In such cases the generator supplier can offer support in this process.

Key points to be within the specification and confirmed within the tender process:

Generator loading schedule
First step load acceptance required of the generator A, kVA, kW, pf
Governing standard to which the load acceptance can be achieved

Other elements of the specification:

When developing the full generator specification consideration needs to be given to the following areas:

Noise and attenuation

Permitted noise level
Provision for sufficient inlet and outlet air whilst maintaining fire break areas
Provision of a route for exhaust gas either through or on the exterior of the building
Isolation of the building from vibration and noise of the generating set

Fuel System

Storage and capacity required
Replenishment of fuel
Fuel polishing
Engine emissions

Other considerations in the selection process:

Ingres and egress of plant into the building
Impact of light load running on the performance of the generator and how this can be mitigated by regular load testing