Path lab gas system guidance reviewed

Reserve supplies

Reserve supplies are not generally fitted to either plant or manifolds. If supplies are critical, alternative sourcing arrangements are usually described in the laboratory operational policy.

Test points

A test point to facilitate engineering and quality control testing, comprising a lockable valve and suitable connector (for example a 3⁄8 inch BSP male fitting), should be fitted to all plant and manifolds upstream of the plant/manifold isolating valve.

Additional purge points to facilitate system testing (for example at isolating valves) may be fitted at the discretion of the laboratory manager.

Alarm systems

Historically, many laboratory gas failure alarms have been sited near the source of supply and hence provide no warning indication in the laboratory. HTM 08-06 stipulates that they should be sited where they can be monitored for at least the period of occupation of the laboratory, for example, in a portering office or the laboratory manager’s office, or some other readily observable location within the laboratory.

Alarm indications presented in HTM 08-06 are shown in Table 1.

System capacity and flow calculations

HTM 22 presented a list of typical laboratory equipment and associated gas usage (in terms of flow and pressures) and detailed diversified flows. Chapter 3 of HTM 08-06 ‘System design’ expands on this list and gives specifications for numbers of terminals, stressing that medical gas terminals should NOT be used for supplying pathology gases.

HTM 08-07 allocates terminals in numbers to equal the laboratory equipment quota, rather than providing an excess in the form of the plugged tees recommended by HTM 22 and moves away from definitive flow diversities. Additionally, whereas HTM 22 specified duplex plant for compressed air and vacuum supplies, HTM 08-06 leaves choice of plant very much in the hands of the laboratory manager, in liaison with the system designer.

Reference is also made to the minimal gas provision (i.e. oxygen, vacuum and air) for a dedicated workshop in which the laboratory equipment might be serviced or calibrated.

Table 1 of this chapter (shown as Table 2 in this article) provides a guide for assessing likely system demands, although with many available equipment types it is stressed that manufacturers’ data must be consulted to complete the assessment.

Testing PLGS

HTM 08-06 presents guidance on both engineering and pharmaceutical testing. Although the installation of PLGS is generally in accord with HTM 02-01, patients are not connected to the systems and, therefore, the extent and depth of testing are less than those required for MGPS validation and verification.

Nevertheless, this reduction must not be allowed to compromise the safety of the working environment that would result from, say, the inadvertent cross connection of flammable and oxidising gases. Therefore, it is particularly important that the tests described are not only carried out meticulously but also documented for examination and retention by the laboratory manager.

Engineering tests

Using oxygen-free nitrogen or helium, each system (with all termination points fitted and isolated) is tested at 1.5 times working pressure. As systems run at 400 kPa or less, all systems are usually tested at 600 kPa. Control devices may need to be removed for this test. Helium should always be used as the test gas for hydrogen systems, as both gases can pass through joints that have been proved leak-tight with nitrogen.

Test criteria

Using suitable measuring equipment, no leaks should be detected during a two-hour test period. Vacuum system pipework is also tested to this standard. No additional testing under vacuum is performed, other than a functional test of the vacuum plant.

Annual leak test

Laboratory managers may request annual leak testing of some (usually flammable) gas systems. For this test to take place, the system must be depressurised by venting off gas in a safe manner, and purged of residual gas by the use of oxygen-free nitrogen or helium. The latter is then used as the test gas.

Cross connection tests

This test is essential to prevent crossover of flammable and oxidising gas systems and is performed using the test gas. The method of test can be that prescribed for cross connection tests for MGPS by HTM 2022 1997 or HTM 02-01.

Design flow and pressure-drop tests