EMISSIONS a case for the FEDs

Global environmental awareness, combined with increasingly stringent legislation, is forcing process plant operators worldwide to identify and eradicate the causes of unwanted and hazardous pollutants, commonly referred to as `fugitive emissions' (FE).

Alarmingly, valve stems and flanges have been identified as being by far the largest cause of fugitive emissions, typically accounting for 60-75 per cent of total plant emissions (European Fugitive Emission Conference 1998).

Historically, valve stem seals and joints were only identified as being a source of unacceptable leakage if it was visible, audible, or the cause of immediate damage or significant process loss. Today, however, against a backdrop of increasing concern for the environment, governments around the world are being urged to produce legislation for controlling pollution levels from industrial plants.

Although the International Standards Organisation (ISO) is currently preparing a standard for FE, there are currently only two legislative bodies governing this area. In 1986 the German government introduced the TA-LUFT Regulations (Technical Instructions on Air Quality Control), specifying statutory limits for fugitive emissions, together with a list of organic fluids considered critical. These instructions also include a maximum permissible emission rate from valve stems.

In 1990 the US Environmental Protection Agency (EPA) introduced limits which, although in principle are similar to the German legislation, use very different methods for measuring and quantifying emissions to atmosphere.

In the UK at the present time, there is no statutory requirement with regard to limitation of emissions to the environment but plant operators are very well aware of the problem and are taking action ahead of any forthcoming legislation. Of course, this action is not entirely altruistic, since reducing fugitive emissions not only has obvious environmental benefits, it can also reap financial rewards in the form of reduced product/process loss and improved reliability and efficiency.

Over recent years, the increased exposure of the causes of FE has bred a nation of more discerning valve users, particularly within the chemical industry. Existing and potential users now seek valves which provide consistently low fugitive emission performance, with added confidence in lifetime in-service performance.

Most of the research undertaken to date into the extent of valve stem emissions has concentrated on the reciprocating stem valve types, such as the gate and globe valve family. Seal developments have given radical improvements in this area. The potential for deterioration of the stem seals in the case of rotating stem designs, such as ball, butterfly and plug valves, is very much lower and therefore so is the danger associated with potential fugitive emissions.

However, manufacturers are nevertheless taking a lead to ensure that their products can be relied upon to provide the security needed by modern environmental considerations.

To enable plant operators to ensure that, once installed, the valves continue to provide emission-free operation, a facility for monitoring the performance of the valve in-line is of obvious advantage - eliminating the time, costs and inconvenience of process or even plant shut-down.

Valve manufacturers, however, must always bear in mind that while valve users seek to fulfil the above criteria, they aim to do so using the most cost-effective, high performance and low maintenance solution.

Valve manufacturers must be able to demonstrate that their products can perform consistently within the required limits of maximum emission, and if required, offer designs which will allow the performance to be monitored during service.

To minimise the possibility of atmospheric leakage from flanged ball valves, manufacturers offer single piece body designs (both reduced and full bore) which eliminate the need for body joint flanges and the potential for emissions from this source.

As with any equipment, there are as many different valve stem sealing arrangements as there are valve designs and the user should be aware of the benefits, or negatives, of the design offered in any particular case.

Stem seals on ball valves usually incorporate PTFE as the primary sealing material. PTFE is inexpensive, extremely chemically resistant, has a high degree of resilience, extremely good wear resistance and has a low coefficient of friction - all positive features for sealing around the rotating stem of a ball valve. However, PTFE has a temperature limit of 250 degrees C and must be supplemented, in the case of fire-safe valves, with a temperature-resistant material such as graphite.

Gland seals should be designed to seal perfectly across the whole pressure spectrum, and even to seal effectively against vacuum conditions. The best designs should also eliminate the risk of over-compression, which could lead to unacceptable increases in operating torque for the valve.

Having chosen a valve design which will provide the required performance level, some users will need to be able to monitor the stem seal arrangement so as to ensure that the status of the gland seal is known at frequent intervals.

A fugitive emission detection (FED) system usually consists of two seal arrangements in series, with a monitoring area between. Manufacturers may supply this either as a bolt-on accessory to a standard valve design, or as an integral design. In either case, it is important that delivery times, QA requirements, spare parts availability, etc, are considered before selection, since these factors can vary significantly between the two.

The availability of FED systems, as bolt-on accessories to standard valve products, enables the user to select or up-grade existing equipment to conform to new environmental standards. PE

Bob Merrifield is engineering manager at Hindle, part of Tyco Valves & Controls