A Case Study in Learning

During a recent CIBSE CPD, we presented on ‘The Safe Use of Gas in Education’ a question was asked about the need for commissioning when our gas control systems were installed.  The engineer asking the question had experience of our systems, understood they were designed to be simple to install and that with an LCD information was displayed clearly for installation diagnostics and user operation.

We spoke of the need for commissioning as laid out within the standards but also explained that for us at Medem, safety is always our primary concern and that within the gas industry we should never assume systems have been installed correctly no matter how simple they are to install.

There are many positives to commissioning, not just confirmation of correct operation.

For instance where our systems are commissioned by a Medem engineer not only do we ensure the items are installed and operating correctly, double our industry leading 5-year warranty to 10 years, provide a 24-hour telephone support number but importantly we also check to ensure the correct system type is installed.

From the original site design to installation a specification can pass through several revisions, a change of design team, take several months and in some cases years before completion on site. This means that the originally designed use of a room or type of installed appliances may differ from the original concept, so the gas safety panel requirements may also now be different.

Classrooms that have changed from laboratories to home economics rooms, classrooms with brazing equipment or kilns that were not on the designs when the gas controls systems were specified and chosen are just a couple of examples.

Because we develop and manufacture our own products we have designed flexibility within the gas safety systems so that we can enable or disable options during the commissioning to reduce or remove potential handover delays.

Although not every site change is as simple as sliding an option switch.

We recently attended site to commission a system within a boiler room of a school which had gone through a partial refurbishment, the old boilers had been replaced for shiny new and more efficient ones.

Due to its rural location and the site being prone to power cuts of fluctuating lengths the specification included for a Medem SEC-B system, detectors and commissioning. For those that don’t know, the SEC-B is a gas detection and gas pressure proving panel specifically designed to help the site meet parts of Building Bulleting 100: Design for fire safety in schools. Monitoring the boiler room for gas leaks but crucially it can automatically restart the gas supply after a power cut.

Once installed the SEC-B was turned on, a downstream integrity test completed and a signal was provided for the boiler plant to start and within a couple of minutes the heating was on everything was operating correctly.

As you would expect part of the commissioning is to test all operations of the gas control panel, so power was removed imitating a power cut. When power was restored the system automatically started a gas pressure test, normally a routine operation for a boiler room. However the panel began alarming and indicated a test fail condition, there was a drop in downstream pressure being registered!!

Now, fortunately, the boffins at Medem have designed in an easy way of displaying the real-time pressures on the display to confirm exactly what is happening. Another unique Medem design feature based on site feedback and experience.

So within a matter of seconds, gas pressures had been viewed and the pressure was confirmed to be reduced downstream of the valve. Now the puzzling part for all onsite was the gas pipework and appliances had only just been tightness tested.

So after retesting and much head scratching what was the cause of this mystery?

A water heater…

Replacing the water heater had not been part of the refurbishment so it was still the original unit. When the ‘power cut’ was replicated to simulate a momentary loss of power the flame safety device within the water heater didn’t respond fast enough in closing its local solenoid. So when the gas test began at startup it showed as a test fail and the gas valve remained isolated.

Replacing the water heater was not feasible due to budget restraints so the question was, how do we solve a problem like time?

The answer?…A delay!!

Such an annoying thing if waiting for a train or plane but ideal in this instance.

With the ability to write our own software and make use of modern electronics in our designs and not just using relays we were able to reprogram an SEC-B, creating a delay before the auto restart. The flame safety device then had time to operate, the gas pressure test was then performed, integrity confirmed and the gas supply and boiler plant were reset safely reducing the potential for cold classrooms or even damage due to frozen pipework.

We have taken the information gained from the latest standards, listening to design engineers and the end user, from onsite commissioning’s and our own experience and filtered this back into our latest range of control panels.

Adding failsafe valve and relay checking, fire alarm and time delay auto restarts are just some of the new developments making Medem systems continue to be the most reliable, safest and flexible available today.

The differential gas pressure proving method as developed and patented by Medem operates by measuring the pressure differentially across the inlet and the outlet of a gas isolation valve using micro transducers. All other systems can only see the gas pressure when the valve is open.

1. Differential proving enables the supply pressure to be measured before the associated isolation valve is opened. This means that in the event of gas over pressure or under pressure the valve would remain safely shut and reported on the system LCD. Therefore only the differential gas pressure proving method can be considered to be truly fail safe.

2. This method is dynamic in that it takes into account any supply pressure changes that could be temporarily created by increased demand such as a boiler firing. Such transient changes in pressure would have the potential to cause other methods to isolate the gas supply needlessly.

3. Some other gas pressure proving methods can fail to identify a gas leak because they have to be set on installation and if the supply pressure at the time of setting is even slightly higher than in subsequent use then a leak can be missed during test.

We are still on occasion asked to supply a system that performs ventilation interlock alone, without gas pressure proving. Although we do have such a system, it was one of our first in fact, we don’t consider it best practice and would always recommend a combined gas pressure proving and ventilation interlock system.


Consider the following…

During normal usage the ventilation goes off regardless of reason, the ventilation interlock system will close the gas solenoid valve, isolating the incoming gas supply just as it should. 
Any kitchen appliances using gas at this time would continue to do so until all the remaining gas (between the valve and appliances) is consumed at which point a flame failure device (if fitted) would isolate the appliance. If the appliances don’t have a flame failure you will be left with an ‘open end’.

But the problem occurs with the fact that you no longer have any gas pressure between the gas valve and outlets.

This would require either a visit from a Gas Safe engineer or a system capable of performing a down stream integrity check.

It is stated in IGEM/UP/1a edition 2 that:
Where a ‘safety shut-off valve’ has been automatically closed, before it can be re-opened, if there is any reasonable chance that the pressure has been lost on the downstream side, a tightness test should be performed before the valve is re-opened.

We feel it’s far the better of the two options to have in place a combined gas pressure proving and ventilation interlock system from the beginning and recommend accordingly.