Which Isolating Valve to Use?

The design engineer is often confronted with a confusion of isolating valve designs - each presented by the manufacturer as the ultimate solution to any given pipe isolation function.

However, closer examination reveals that some designs are more suited than others for specific applications. Enclosed below is a brief assessment of the best suited isolating valve for various pipeline functions.

Pump Suction Isolation Valves
Most pump problems encountered are generally as a result of poor suction design. Of fundamental importance when designing a pump suction is to ensure a smooth inflow to the pump impeller. This implies a straight pipe length approach of six to seven pipeline diameters and minimising any vortex action that may be caused by valves or bends.
Butterfly valves are often used as cost effective pump suction isolators but the presence of the disc in the flow stream creates vortexes which impact on the pump NPSH behaviour, power output and general pump performance. In addition, butterfly valves have no indication of whether they are open or closed. Starting a pump against a closed suction isolating valve can cause damage to the pump.

Gate valves are more suited as pump suction isolators because the full flow area of this design offers no resistance to flow thereby preventing turbulence in the suction pipe.
A rising spindle gate valve (generally a wedge gate valve) is preferred by pump operators because the operator can at a glance see if the valve is opened or closed before pump startup.

Pump Discharge Isolators
Pumps are normally started against a closed isolating valve. In addition an isolating valve is installed subsequent to a pump check valve to allow for pump maintenance.
Gate valves are traditionally utilised for this application. Resilient seated gate valves can be used for low head and small diameter applications but are not suited for high pressure and/or large diameter applications.
Metal seated wedge gate valves should be utilised in applications where pressures exceed 10 bar and where diameters are larger than DN300. There is a growing trend to utilise butterfly valves in large bore applications. Decisions to utilise either butterfly valves or gate valves are dependant on initial cost, flow and pressure and, the design engineer's experience.
Isolating Valves along the Length of the Pipeline
The decision as to which isolating valve design to utilise along the length of a pipeline is dependant on cost, pipeline diameter, and pipeline pressure and pipeline material. The decision can be simplified as follows:

Small Diameter Low Pressure Pipelines
Resilient seated gate valves are the most cost effective and reliable choice for pipelines up to 300NB and for pipeline pressures of up to PN16.

Large Diameter Pipelines
The choice between butterfly or gate valves for large diameter pipelines are based primarily on cost, pressure rating and whether the pipeline will be pigged or not.
Single and double offset butterfly valves are extremely cost effective for low flow and low head applications and are most often chosen as isolators for this reason.
Some pipelines are pigged frequently due to nodular growth in the pipelines. In applications like this, gate valves are the logical choice as butterfly valves would prevent the pig from being drawn through the pipeline.
A wedge gate valve because of its wide selection of seats, positive guidance through its stroke and wedging action on closure, tend to provide a more positive seal to triple offset butterfly valves in high pressure applications. In addition, wedge gate valves are often more cost effective than triple offset valves in these applications.

Scour Valves/ Line End Valves
There is a growing trend to utilise Resilient Seated Gate Valves (RSV's) as scour valves. However, this design of isolating valve is not suitable for this application as the gate is coated with rubber which requires cool and damp conditions. The rubber coating in this application either perishes or, is attacked by rodents, causing the valve to leak. In addition, neither RSV's nor butterfly valves can withstand the high differential pressures and high velocities across the valve under scour conditions. The metal-to-metal seal of a wedge gate valve is not affected by exposure to the elements or by rodents.

This design is also guided throughout its stroke; its operation is therefore not affected by high scouring velocities and high differential pressures.

Air Valve Isolators
RSV's are cost effective air valve isolators in low pressure applications. This design however, is limited in pressure range resulting in the use of butterfly valves or wedge gate valves in high pressure applications.

Butterfly valves are not suitable as air valve isolators because the disc in the flow arrangement of the butterfly valve design creates turbulence and restricts the air valve's performance capacity.

Wedge gate valves are most suited as high pressure air valve isolators because this design provides a full flow area and full guidance across its stroke, and the wedging action on closure provides driptight shutoff for air valve maintenance purposes.

Conclusion
Isolating valves are either open or closed and are seldomly utilised for regulating purposes. RSV's , wedge gate valve and butterfly valves are the most generally utilised forms of isolating valves for water pipelines. Cost, function and pressure and size range are the determining factors when selecting an isolating valve. Wedge gate valves have, due to the marketing effort of RSV and butterfly valve manufacturers, lost popularity over the past twenty years. This document indicates that there are some pipeline applications that are only suitable to this design. It is imperative therefore for the design engineer to not only take into account
the initial cost of the valve but, the cost of ownership i.e., the cost of maintenance and/or replacement of the valve over the lifespan of the pipeline.Isolating Valve To Use?