TECH TALK
Ensure Your New Valve Station is Well Thought Out
Control valves are pivotal components of municipal water systems , playing a crucial role in regulating water pressure and flow to ensure a stable supply . Their widespread presence in these systems highlights their critical role in the operational integrity and dependability of water distribution networks . Selecting the appropriate valve size is not only essential for optimal functionality but also for the longevity of the valve ’ s service life .
By Mark Gimson , Director of Marketing and International Sales – Cla-Val
It is equally important to consider the working environment for the personnel tasked with valve maintenance , ensuring it is conducive to efficient and safe operations . Proper sizing of control valves is paramount , as it directly influences efficiency , control precision , safety , and cost-effectiveness of the water distribution system . By adhering to these best practices , control valves can deliver years of reliable service , contributing to the seamless operation of water distribution infrastructures .
A well-designed valve station requires planning that considers the following :
1 . Sizing your valve
Most valve stations in municipal water situations are pressure-reducing valves . Selecting the right size is important , as problems can arise from incorrect sizing . Rather than sizing the valve based on the size of the line , it is necessary to consider flow , pressure , and velocity . Knowing the size of the water main , and the maximum , average , and minimum flow demands , one can move forward in the valve size selection . Some manufacturers have a sizing software program that makes it quick and simple to size a valve correctly but understanding what the program is considering is useful .
A sizing software program will select a valve size based on the maximum required flow the valve has to pass , based on the minimum system differential pressure . This will be the upstream pressure minus the required downstream pressure that the reducing valve will be set at . The program will consider the velocity passing through the valve ; it is best to limit this to 20 feet per second ( 6 m / s ) in valves operating continuously . In valve stations , it is common to have two control valves in parallel . The larger valve on the main line is designed to take care of peak flows , while the smaller second valve on a bypass will take care of low flows . Typically , the smaller bypass
Accessory packages like the Cla-Val Model XP2F is a complete instrumention accessory package
A well planned out valve station , has good access for maintenance
valve will be sized to handle approximately 20 % of the peak flow .
The smaller valve is also set slightly higher in pressure than the large valve . This allows the smaller valve to open first in the sequence , followed by the large valve as flow increases . The smaller valve is also the last to close as flow diminishes . This bypass line also offers redundancy as either of the valves can be used to supply the network while the other is being serviced . This is particularly important where it is not possible or practical to shut off the main water supply for periods of time .
Some manufacturers have software to support valve station design and allow engineers to consider the impact of velocities , among other things . Often , the velocity in a bypass valve can be excessive , potentially causing damage to the small valve and increasing loud noise levels . Using software , a designer can specify the use of a transition device that will bring the main line valve into operation before high velocities are reached , which is often before the lower pressure setting of the larger valve .
2 . The risk of cavitation
Anytime there is an application where reductions from a higher pressure to a lower pressure , and that ratio is greater than 3:1 , there is a real risk of cavitation .
This phenomenon will destroy a valve over time due to erosion and can create noise and vibration in the pipeline that may be a problem for nearby residents . However , unlike in days past when multiple valves would have to be installed to stagger the pressure drops , this can now be achieved through an anti-cavitation trim . This is a specific trim consisting of two stainless steel cages with slotted openings or drilled orifices that contain the cavitation in the center of the cages , eliminating disastrous erosion issues .
It is always recommended to run a cavitation software program to determine if cavitation will be a problem , and this should be a program specific to the manufacturer ’ s valve . Generic programs are available but often do not account for the actual body shape of the valve .
3 . Options to customize
The real advantage of diaphragm-actuated globe valves is that one can have a pilot system tailored to a specific application . Numerous features can be added to give a custom solution that provides real operational benefits .
Stainless steel pilot systems are popular and the price differences to brass / bronze or copper is not prohibitive . In addition , it should last longer , particularly in harsher environments . Pilot systems can be installed on either side of the control valve , so give some thought both to the location for accessibility with ladders and also clearance for maintenance . Pilot tubing is frequently stepped on or knocked because of poor placement – with a little forethought , this is avoidable .
The pilot system should always be installed with a strainer . This can either be a flow clean , internal style , or an external Y-strainer style . In tough applications , a water filter can be used . Pilot isolating valves are vital for maintenance and troubleshooting of the valve – even on the smallest of valve sizes .
There are other options to consider . A valve position indicator is very useful for operations personnel as it gives the ability to see the exact position of the valve during operation . If a remote signal is required this can be upgraded to a position transmitter supplying a 4-20 mA signal via SCADA , or a limit switch can be added to give an output for a position transmitter . Pressure transmitters can be added to give 4-20mA signals for both inlet and outlet pressures . This option is useful if the intention is to utilize a pressure management program in the future .
Also , consider options to provide more system control . A second pilot control can be added to give the ability to sustain upstream pressure . This is very useful in situations where there is a need to protect the upstream from being starved of pressure by high downstream
32 Valve World Americas | October 2024 • www . valve-world-americas . net