Feature Boilers, pumps & valves The bottom line of steam control
Poor control, reduced life span and potentially hazardous situations are often brought about in valve applications by overlooking one of the most critical components within the steam and condensate system. The Steam Control Valve is one of the most critical components within the steam and condensate system, controlling pressure, volume or flow direction in a fluid transmission system. Marc Bozdogan, managing director of independent steam control specialist Valveforce has more on why this component should be given more attention than it normally gets
designed with a unique ability to con- trol process temperatures and pres- sure. However, there are some very important factors to take on board in order to optimise performance. To maintain a good steam system, keeping steam dry and free from mois- ture is essential, because condensate in your system can cause problems such as water hammer and corrosion. Water can also act as a thermal insula- tor, reducing heat transfer rate. Pipework layout is also important –
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arguably best to use an effective sepa- rator to ensure dry steam. If this isn’t possible, then at Valveforce we would install, at minimum, a condensate line drip pocket to prevent condensate building up in a system. The commis- sioning stage just after the installation is the most critical time to assess per- formance- and here, surprisingly often with steam systems, we see all manner of solids within the pipework. These unwanted objects can end up stuck in the control valves, especially on the seat, causing physical damage to the valves and potentially make them fail. It is not that much of a shock for a
Valveforce engineer to find welding rods, nuts and bolts – and even the odd drinks can in the pipework, so as a rec- ommendation, it would be wise to install a fine mesh strainer on its side to avoid the build-up of condensate slugs. From our position as an indepen- dent valve supplier which also designs
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team control valves, which are used throughout industry for mixing and isolation, as well as flow and modulating control, are
and installs bespoke steam and flow control systems, we find that the majority of steam control valves are used to control pressure and tempera- tures. By reducing the steam pressure, the steam volume naturally increases, which in turn leads to increased steam flow rate velocities. Expanding the discharge pipe size by at least one or two pipe diameters will reduce the velocity and noise – and prolong the life of the system. Beyond the control valve, I’d recom-
mend leaving a run of at least 10 pipe diameters before the inlet connection to the heat transfer process. This will bring about efficient laminar flow, as well as avoid fluctuations and turbu- lent flow in order to get the most energy from the steam itself. This can be doubled to 20 pipe diameters if the application is pressure reduction. At Valveforce we are often asked
what to do about the risk associated with control valve loops in case of a shut down – whether it is planned or unplanned. Our answer is that if your plant is critical, that you need to double up on the equipment to ensure 100 per cent up-time, but realistically we would recommend a bypass isolat- ing valve with a regulating cone fitted so that at least you have some kind of manual control in the unlikely event of a failure.
Installation guide When installing Steam Control Valves, operators need to adhere to the term ‘Flow Coefficient’, which was first introduced in 1944 as an international reference for valve capacity. By defini- tion, the valve flow coefficient ‘Cv’ is the number of US gallons per minute of water at 60˚F that will pass through a given flow restriction with a pressure drop of one psi. As part of your installation guide,
leakage rates are also essential because the majority of valves are designed to
A Valveforce installation at dipping plant specialist Greenbrook
Top left: Severe service control valves are advised for steam and water applications such as turbine bypass
A Valveforce ball segment valve
have a specific internal permissible leakage rate standard (FCI/ASNI). Essentially there is a range of I to VI where the higher the number the tighter the seal. For steam applications, a Class IV should be specified on a mod- ulating control valve and a reinforced PTFE stainless class VI shut off might be required for high integrity isolation. The term ‘Range Ability’ is given to
the ratio between the maximum and minimum ‘controllable’ flows through the control valve. The minimum con- trollable flow is not the leakage flow rate (which is allowed in accordance to FCI/ASNI standards) but the mini- mum flow that is still controllable, and that can be changed up or down as the valve is controlled and throttled. And finally Valve Noise, which must be controlled properly. It is recom- mended that globe control valves for steam services should not exceed 85 DBA, because a high DBA implies high and excessive
outlet velocities. Not only is this a dan- gerous sound level but it also shortens the life of the valve due to high veloci- ties and vibration. Special low noise trims and pipe size calculations can help avoid these problems. Valveforce T: 0121 7111 908
www.valveforce.co.uk Enter 314
SEPTEMBER 2013 Factory Equipment
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