Design of Valves, Calculation
The design of our control valves comes from our theoretical and long-term practical experience in operating and servicing valves. During our calculations we also take into account the suitability of usage of individual valve constructions for a given application. For the calculation of control valves we primarily use the program CONVAL from F.I.R.S.T. GmbH Company, which contains a database with most types of valves from significant producers.
Optimization of Control Valves (Gain, Sensitivity)
The suitability of a control valve for a given application is taken into consideration, not just from the perspective of the value of the flow coefficient, Kvs (Cv), but also in consideration with other aspects, such as gain and sensitivity.
The simple definition of valve gain is the ratio between changes in the flow rate to the appropriate change of the valve stroke.
Custom valve characteristics are determined at a constant pressure drop across the valve. In reality, the pressure drop across the valve varies, which is related with characteristics of the pumps connected to the valve.
By increasing the valve lift, pressure loss decreases; therefore, the resulting characteristic of the installed dynamic valve is different from its custom characteristics.
When selecting a control valve for a given process, it’s necessary to take into account the installed characteristics. The parameter, which is called gain, is a function of the custom valve characteristic and is a specified process condition.
For the specified operating range of a control valve, its value of gain must be in the range of 0,5 to 2 (recommended by EnTech).
In general, this applies to the value of gain, which can be achieved in the widest control range using globe valves (up to 60% of its control range); the narrowest range shows control butterflies (up to 20% of its control).
So, the correct selection for a type of control valve and its characteristics, with respect to the specified process conditions, significantly affects the stability of the control loop and the possibility of simpler tuning.
CONVAL, the computer program we use, simulates the characteristics of the pump from the specified process data. Combined with custom characteristics of the selected valve, it calculates the values of gain in each point of the control range.
Calculation, choice and supply of valves are based on our experience and knowledge of products from renowned manufacturers. A portfolio of each valve manufacturer is based on different, mostly historically relevant, areas of industry. Therefore oftentimes, with big deliveries it’s really beneficial to combine the advantages of multiple valve manufacturers so as to focus on the exact needs of a certain application.
Providing qualified service is, not only a necessary part of sales, but also a source of practical experience for us. Thanks to service we are able to compare theoretical knowledge with real conditions and actual behaviors of valves in various applications. Also, we apply this acquired knowledge when choosing new valves.
The diagnostics of valves is a very important part of our services. Thanks to diagnostics we can identify malfunctions or possibly defective valve parts without the necessity of removing the valve from the pipe. Also, thanks to diagnostics, it’s possible to detect changes in the behavior of valves and thereby predict oncoming malfunctions. In this case we are talking about predictive valve maintenance.