”Control valves are one of the most common, but also most important, instruments in the plant. By regulating the fluid flow in connected pipes, valves can achieve desired operating parameters, thereby ensuring smooth and efficient operation.”
Group product management executive at Spirax Sarco, James Bozward, encapsulates why it is that in so many industrial processes, the identification, purchase and maintenance of valves is of paramount importance.
It’s a stance with which Vincenzo Sipala, severe service solutions manager, Emerson Automation Solutions, concurs.
It is critical to understand the specific application and select the right valve solution accordingly
Vincenzo Sipala, severe service solutions manager, Emerson
Control valves make one of the largest contributions to process plant efficiency. Incorrectly sized or selected, they can have a significant impact on safety, reliability and efficiency, and can cause a slowdown or shutdown affecting profitability.
“It is critical to understand the specific application and select the right valve solution accordingly that will manage potentially dangerous phenomenon such as cavitation, flashing, outgassing, gas noise, suspended solids and vibration,” says Sipala.
“Repeated studies in the process industries show that 30% of control loop variability is caused by poor valve performance and/or tuning. This can be eliminated by correct sizing, selection and setup of the control valve.”
How though does one make the right choices when purchasing and installing new equipment?
Bozward’s checklist for valve selection is:
• Process stability
• Material suitability
• Relevant approvals
• Integration/communication with site infrastructure.
Of course engineers answer to managers – with less technical expertise and a focus on the bottom line – for whom purchasing price, running costs and energy savings may loom larger.
Responds Bozward: “It’s crucial to look beyond the initial purchase price as process is key and should therefore be prioritised. Users should investigate the hidden costs that can impact a given process throughout its lifespan.”
This can be achieved, Bozward advises, by reviewing a valves total cost of ownership (TCO) that considers the capital cost of purchasing the valve, the cost of annual maintenance activities, and any efficiency savings the control valve can provide.
Spirax Sarco’s Spira-trol control valve range [see image below] is one example, focusing on reliability, simple commissioning, ease of maintenance, wide choice of options and support.
Valves can be maintained without being removed from the pipeline and their components can be replaced without the need for special tools.
Advises Sipala: “Correct valve selection is crucial because any oversized or undersized valve can impact process variability and increase plant costs. A poorly performing valve forces the valve, control loop and plant to work harder and less efficiently.
“This leads to greater maintenance and higher operational costs due to off specification product rework and/or greater energy consumption.”
Therefore, select a valve assembly that produces lower process variability in order to reduce potential plant upsets and maximise plant profitability.
Key considerations are to beware of incorrect valve style (making control difficult over the range of use), valve sizing resulting in high friction, plus poorly constructed control valves incapable of required performance.
Having an effective regime in place enables one to efficiently predict many performance issues. Complaints about the cost of a correctly selected valve can be countered, says Sipala, by highlighting that a day of lost production at a refinery or petrochemical plant can cost the end user hundreds of thousands of pounds.
“Ideally the vendor should encourage the customer to understand the importance of correct valve selection from a broader financial impact point of view. Control valve manufacturers should be considered more like consultants than simply vendors,” says Sipala.
Action not reaction
If replacement is not an option, proactive rather than reactive maintenance of existing valves and actuators is key.
The ability to identify subtle differences in operating characteristics between a fully functioning valve compared to one that requires maintenance ensure one is better able to predict when to act, says Bozward.
“Simple things to remember to help reduce maintenance requirements include making sure the internals of a control valve are made from quality materials, which will help mitigate against wear and tear.”
Stainless steel is a good example, as it can protect against the effects of even highly corrosive steam.
Simple things to remember to help reduce maintenance requirements include making sure the internals of a control valve are made from quality materials, which will help mitigate against wear and tear
James Bozward, group product management executive, Spirax Sarco
“If a control valve is too large for a given duty, it will operate more closely to the seat and therefore experience more wear. Similarly, incorrectly sized valves may also be subject to cavitation and noise in the flow, which makes trim components an important factor to consider,” explains Bozward.
“They can reduce noise emissions and anti-cavitation because the large gallery area reduces flow, velocity and noise, providing an overall improved working environment. The use of smart positioners, meanwhile, can aid accurate control valve positioning.”
Correct selection guarantees efficiency and saves cost, reminds Sipala. A properly performing valve lasts longer, while a reliability-centred maintenance programme monitors daily performance for any signs of change.
Emerson’s FIELDVUE digital valve controller, connected to an asset management system such as AMS, enables real-time monitoring and highlights performance degradation and/or maintenance needs.
There’s a safety benefit too: studies show that 60% of safety- related incidents occur during reactive maintenance.
Finally, take a holistic approach. Effective flow, especially in high-pressure environments such as oil and gas, depends on the interplay of control and ball valves. Emerson Automation Solution’s new Fisher Z500 metal-seated ball valve’s bi-directional sealing design offers a solution to process back pressure and shutoff requirements in both flow directions, safeguarding control valves and other equipment in the line.
Designed with an integral metal seat and self-energised metal body gasket to help eliminate leak paths and withstand extreme temperature changes, the product’s live-loaded packing, side-mounted brackets and fixed centrelines help reduce side loads and decrease overall wear.
Kai Feller, product management and marketing manager at Festo, offers his advice when purchasing valve equipment for flow control...
Start with the media to be controlled, says Feller, because it will affect the options available for the material of the valve. Then determine the correct construction material.
“If the media is aggressive or corrosive, then stainless steel is likely to be the best option. However, for many common applications – supply of cooling water in a machine for example – a brass ball valve or butterfly valve with polyamide coated discs is perfectly adequate. They also cost far less than stainless steel.” Operating pressure can also have significant cost and energy implications.
All valves are pressure rated: if the process pump supplying the media will only ever deliver at 4 bar, or if the valve is mounted on the outlet of an open vessel, explains Feller, tone can select lower media pressure ratings on the valve.
The connection norm and the burst pressure rating of the valve remain unchanged. In turn, a lower pressure rated disc leads to a smaller, lower cost actuator.
When using pneumatic actuators, ascertaining the compressed air network pressure is critical: “Industrial applications commonly use 5.5 or 6 bar systems as they are believed to be the most economical in general terms.
“If the customer is running on a different pressure, the actuator size might need changing because its size needs to be proportional to the pressure in order to avoid compromising the necessary safety factors,” he says.
There may be an opportunity to use a smaller actuator or a local pressure booster with potential cost and energy savings. Additionally, specifiers can optimise control while also cutting cost.
Many electro-pneumatic positioners with numerous features can work in the toughest environments as standard but, Feller warns, ATEX compliance comes at a price.
“If your operation is not subject to extreme conditions, then look for a cost-effective alternative which offers the same core control functionality without the Ex rating. If the application is even simpler, then a 5/3 control valve with analogue sensor box is an adequate and low cost option.
“For larger installations, a valve terminal can supply multiple process valves and has fieldbus and communications connectivity built in. Specifying valve terminals reduces wiring requirements.
“In addition, valve terminals give visual status confirmation, diagnostics and functions, like manual override, at a convenient location which make commissioning and maintenance much easier because the controls are readily accessible.”
Savings can be even more significant once cost for installation, wiring, loop-checks and start-up or even maintenance are taken into account
Kai Feller, product management and marketing manager, Festo
Basic valve terminals can be very cost effective depending on the number of valves to be controlled. Savings can be even more significant once cost for installation, wiring, loop-checks and start-up or even maintenance are taken into account.
Questions are sometimes raised regarding the compatibility of pneumatic actuators and process valve movements, acknowledges Feller.
“Pneumatic actuation enables rapid changes which can help to improve cycle times, but this is not something that is always welcome in process control.”
Controlling the air flow to regulate speed and smoothness of operation is therefore important, he explains. Flow control valves can throttle the flow in and out of the actuator. Where the reduction in flow of the inlet air might lead to slip stick effect, throttling of the outlet air can be difficult to achieve.
“Where single-acting actuators are used with a state of the art Namur mounted solenoid valve, the rebreather function will make the valve faster if the exhaust port is throttled.
“The main purpose of the rebreather function is to supply the spring chamber with clean process air instead of dusty and humid environmental air, reducing wear and corrosion. The correct way to slow down the speed of these valves is a flow control plate, which is available with one-way or two-way control.”