The art of interpreting specifications to ensure correct valve design

When a control or choke valve is specified, the data sheet – naturally – is prepared to reflect the running conditions that the valve will be required to cope with. If the valve is used on a severe application, it might have to cope with many varying conditions including erosive fluids, high trim velocities, high temperature or high pressure. There might also be fast acting, anti-surge considerations which also need valve stability under modulation, unspecified start-up or abnormal conditions. The data sheet may well specify a complete range of conditions for the valve to cope with... in many cases it does not! Sometimes the service is not clear, being specified only as ‘Gas to X100’.

It takes experience to interpret these specifications, to determine complete operational requirements for the valve, to understand the application and translate them into a valve solution that is truly fit for purpose. Failure to do this correctly could easily result in an incorrect specified valve.

At Koso Kent Introl, the project engineering team knows that, only if the full set of data is taken into consideration, can the valve’s complete ‘purpose’ be known, and the correct valve proposed. Through hard-won experience in the field, they know that they cannot simply look at given data at face value and begin designing a valve solution against that. The KKI team will seek to determine all the modes of operation that the client really wants the valve to operate under; they will not (as others might) simply quote against the spec sheet.

For instance, for a valve within a water injection well specification, normal running will be shown on the data sheet (e.g. a drop of 3000psi to 2500psi). However, the water injection pump may be 2km from the well. Therefore, at start up, with an empty line, the pressure drop is then 3000psi to 0psi. This may only be for a short time, and it might only ever occur very infrequently. But if not taken into consideration, this scenario, when it does occur, could be catastrophic for the valve, if it is only designed for the running conditions given on the original data sheet.

Take another example: this time in the power industry, within a turbine bypass application. If the turbine trips, steam must be released immediately, opening the valve in parallel with the turbine. If this doesn’t happen quickly enough, the safety valves may lift instead. However, these will often not re-seat without some maintenance effort which will result in unscheduled downtime. Since the steam produced by the boiler feeds other critical processes, the consequential effect could be extremely costly.

There are many similar such applications which require a full understanding to specify the valve correctly. In critical processes and severe service applications, it’s the potential, extraordinary events with which the KKI team are concerned, in addition, of course, to those conditions specified as ‘normal’. They are only too aware of the implications and consequences of a valve failure and will not compromise as they prepare their design. The KKI design and support teams understand fully such operational challenges and potential problems for any specific application. Their aim is to eliminate these through good design. Operational experience is essential to achieve this.

However the team is also totally aware of the cost implications of over-specifying a valve solution. For instance, KKI does not automatically apply strict trim exit velocity criteria to their designs, as sometimes happens elsewhere in the industry. Whilst recognising the importance of trim exit velocity, they are also aware that there are many other factors which also need to be taken into account. The KKI approach ensures that the solution is designed to give the best commercial solution whilst ensuring the valve will work effectively under all required conditions.

The team is sometimes called upon by a customer to solve a particular operational problem with a control or choke valve. On many occasions in the past four decades, the team has been asked to help to design new solutions after original suppliers have failed to accommodate the full set of scenarios that a valve might face.

Sometimes conditions change with time and become more demanding, or a particular valve is incorrectly specified initially, leading to operational problems insitu. In such circumstances, it may be possible for the team to design a KKI Vector labyrinth trim to replace the supplied trim. This solution eliminates the need for replacing the expensive, long lead-time valve body, since the fault in most cases lies with incorrect trim design. This could save at least 40% versus the cost of replacing the whole valve (or more in the case of very large valves; less if the valve is less than 6”), and probably at least halving maintenance downtime. In such instances, KKI will then guarantee the whole valve as a single new entity, irrespective of the original manufacturer.

As the KKI team knows, the key to getting the solution right relies on building a good understanding not only with those specifying the valve – but also with those who will be operating it over many years. Often these seem to be competing interests, but the KKI team is proud of its reputation for striking the right balance, and for delivering a quality, cost effective valve solution – over the whole life of the project and the valve.

Koso Kent Introl
Tel: +44 (0)1484 710311
www.kentintrol.com

Published: 28th June 2013