Design engineers are frequently confronted with the fact that they have to decide on the best type of isolation valves to use noting that each manufacturer presents their design as the best option.
However, on further analysis and examination, it has been noted that some valves tend to be better suited to a particular application. Below is an assessment of the isolation valves suited for a particular pipeline function.
Pump Suction isolation valves
Often pump problems are as a result of poor designs of the suction. It is fundamental for the pump section to allow a smooth inflow of the liquid into the pump impeller. To get the smooth flow, it means that 6 or 7 pipelines with similar diameters are connected with few valves or bends to avoid affecting the flow.
Often butterfly valves are used as pump suction isolators. However, using the valves is not recommended due to the discs which create vortexes that affect the pump’s NPSH behavior, performance, and power output. Furthermore, with the butterfly valves, they do not have an indicator to show when they are open or closed. The lack of an indicator may result in damages to the pump in case the pump is started, but the valve is closed.
The best option for a pump suction isolator is the gate valves. The valves have a design that does not offer any resistance to the flow of the liquid thus preventing turbulence that affects the drainage system.
Rising spindle gate valves are also preferred because the operator can easily note whether the valve is closed or open before the pump is running.
Pump Discharge Isolators
To start a pump, it is started against an isolation valve that is closed. An isolation valve is fit after the pump to allow pump maintenance.
For this application, the gate valves are the traditional types used. The most resilient gate valves are used in the low head and small diameter situations but not in high pressure or large diameter situations as they are not suited.
For situations where the diameter is larger than DN300 and a pressure of more than 10 bar is involved, metal seated wedge gate valves are the best option. The option of using the butterfly or the gate valves is solely dependent on the flow and pressure, costs and lastly the engineer’s experience and preferences.
Isolation valves for the length of the pipeline
To decide the type of isolation valve to use along the pipeline’s length, there are four main factors affecting the decision; the cost of the service and the diameter, pressure, and material of the pipeline. To simplify the decision, note the explanation below:
Small Diameter Low Pressure
For pipelines of up to 300NB and pipeline pressures up to PN16 would be fitted with resilient seated gate valves as they are the most reliable and cost-effective.
Large Diameter Pipelines
To decide between the gate and butterfly valves for the larger diameters, you will have to compare costs, the rating of the pressure and whether pigging will be done on the pipeline or not.
Applications with low flow and head applications should use the single yet double offset butterfly valves. The butterfly valves are very cost effective for such applications and are often used as isolators.
Pipelines that are pigged regularly as a result of nodular growth should have gate valves fitted inside the pipes instead of the butterfly valves. The butterfly valves are avoided as they inhibit the drawing of the pig through the pipeline.
Wedge gates provide a more secure seat to the triple offset butterfly valve designs when high pressure is involved. The wedge gate is preferred in such applications due to its wide range of seats to select from, its welding action when closed and lastly how it positively guides through its stoke. The wedge gate valves are more effective as compared to the triple offset valves in the high-pressure applications.
Line End Valves/ Scour Valves
Recently, CGIS Valves are being utilized as scouring valves. However, this tactic is not recommended as the gate has a rubber coating which needs cool and damp conditions otherwise it is attacked by rodents or perishes resulting in leaks on the valve. Furthermore, using the Isolation Valve Supplier – Zero Leakage Isolation Valves | CGIS offers and butterfly valves to withstand the high differential pressures and velocities especially in scour conditions is not recommended as they can not withstand the conditions.
For a more resistant valve to the elements or rodents, opt for the metal to metal seal of the wedge gate valve design. Furthermore, its guide through its stroke makes it resilient to the high scouring velocities and differential pressures.
Air Valve Isolators
RSV’s are more appropriate for use as air valve isolators for only low-pressure applications. High-pressure applications will require the use of butterfly or the wedge gate valves.
A simple explanation as to why the butterfly valves are not efficient has been explained above; they have discs in its arrangement flow which creates turbulence that affect the performance of the air valve.
The high-pressure applications for air valve isolators can only be done by the wedge gate valves. Their use is efficient due to their full guidance on their stroke, the full flow area and the drip tight shut off due to its welding action. These characteristics make it an efficient air maintenance valve.
Isolating valves should rarely be used for regulatory purposes, and they are either closed or open. The butterfly, wedge gate, and RSV’s valves are the most commonly used for the water pipelines as isolating valves. To select an isolation valve, the four main factors considered are size range, pressure, cost and function of the valve. Over the years, the wedge gate valves have reduced in popularity due to the extra marketing effort by the butterfly and RSV valves manufacturers. In this article, the various designs and preferable pipeline actions that they work most efficiently in are elaborated. Also, the design engineer should not only take into account the valve’s initial cost, but they should also take into account the maintenance or replacement costs over the pipeline’s lifespan.