Sizing, designing and commissioning control valves
22 May 2014
Authors: Tom Scanlon, process engineer, and Samuel Solomon, student process engineer, Ipsen Manufacturing Ireland
The Chemical and Process Engineering Division of Engineers Ireland hosted a seminar on ‘Type and Sizing of Control Valves’ on 3 April at UCD’s School of Engineering. The guest speaker at this seminar was Martin Bruessau, the managing director of Samson Controls Ltd, UK.
Due to the variety of types of control valves like globe valves, butterfly valves, ball valves and rotary plug valves, the right selection becomes difficult. The selection criteria were identified and the pros and cons were explained, with recommendations for practical use. With regard to the sizing of control valves, these are sized in accordance with DIN EN 60534. An analysis of key factors identified critical-flow conditions as cavitation.
The objectives of the seminar were to:
- Outline selection criteria needed to choose the appropriate control valve for a given application;
- Give an overview of different control valves and their respective characteristics and working mechanism. This included a list of advantages and disadvantages for each control valve;
- Compare valves in terms of pricing and sizing; and
- Outline the importance of appropriate sizing of control valves and analyse parameters needed in sizing a control valve.
SELECTING A CONTROL VALVE
By giving an initial overview of the selection criteria needed in choosing a control valve, participants were able to understand the importance of gathering relevant information about an application from the start. To further emphasise the importance of information gathering, some of the consequences in choosing the wrong control valve were highlighted.
A systematic approach was recommended to minimise the risk in choosing the wrong control valve. A datasheet details all the respective criteria needed in selecting a control valve for a given application. Process data included are:
- Flow rate;
- Inlet and outlet pressure;
- Area of installation;
- Specific data of the fluid (density, viscosity, etc);
- Sizing (Kv values, characteristic size, noise levels etc); and
- Leakage (internal and external).
An overview of different control valves was presented, categorising different types of control valves into two major groups.
- Group 1: Linear Movement – e.g. globe valves, gate valves and diaphragm valves
- Group 2: Rotating Movement – e.g. ball valves, butterfly valves and rotary plug valves.
Each of the listed valves was characterised and the working mechanism was briefly explained. This also included assessing the main advantages and disadvantages of each valve type. Globe valves are superior to all other valve types in terms of precise control of flowrate. Emphasis was also placed on the usage of butterfly valves, which should be used in applications where precise control of flowrate is less important.
From a price chart displayed in the seminar, it could be seen that the price difference between valves of different types decreases with a decrease in nominal valve size. However, as the nominal size increases, the price difference between valves rapidly increases. For larger nominal valve sizes (i.e. above 150 DN), it was evident that the globe valve is the most expensive and the butterfly valve the least expensive.
An important point made was that larger flow rates do not require precise control. Thus, unless control is an absolute necessity, using cheaper end-valve types (which have wider tolerances) such as the butterfly valve should be given consideration.
An aspect considered in this seminar was the sizing of control valves, which sometimes is neglected in the industry. Engineers tend to put more consideration into budget and control and less consideration into the potential size and weight of the control valve. For example, globe valves are very precise in controlling the flow rate but have a much larger size profile than other types of control valves for comparable flow conditions, due to their working mechanism.
To design a control valve for a given application, the valve should be correctly sized. This is an integral step as the control valve selected needs to be able to handle the flow without sustaining damage or damaging the pipe.
IMPORTANCE OF THE Kv VALUE
For practical use, to select the appropriate size, the minimum and maximum flow capacity represented by the Kv value for the defined working conditions is to be calculated.
The Kv value is a coefficient and it is defined as “the characteristic value of the flow capacity corresponding to the flow of water through a valve (m3/h) in conjunction with a differential pressure of 1 Bar”. The set of Kv values obtained need to be within the range (Kvs:Kvr) specified on a chosen valve. I.e. the maximum Kv value calculated needs to be below the specified Kv value on nominal stroke (Kvs) and the minimum Kv value needs to be above the lowest Kv value controllable (Kvr), taking a 10% safety margin into account in each case.
Another issue highlighted by Bruessau was the danger of critical flow conditions such as flashing and cavitation. These phenomena occur when the vapour pressure of the fluid is between the lowest working pressure and the outlet pressure (cavitation) or when the fluid’s vapour pressure is above both the lowest working pressure and outlet pressure (flashing).
As a result, the fluid vaporises (flashing) and additionally implosions may occur (cavitation). This consequently damages the valve and or pipe and in the case of cavitation high noise levels are generated. To prevent this issue, it was suggested that special anti-cavitation/flashing trims should be used.
In conclusion, this seminar was very informative to both student and professional engineers. Particularly useful was the comparison of different valve types and the emphasis placed on the importance of choosing the right valve type, not only from a price perspective but also in terms of the size profile. Also very useful is the relation of theory and practice. This is very important to bridge the gaps between design and end-user applications.http://www.engineersjournal.ie/2014/05/22/sizing-designing-and-commissioning-control-valves/http://www.engineersjournal.ie/wp-content/uploads/2014/05/Sam.jpghttp://www.engineersjournal.ie/wp-content/uploads/2014/05/Sam-300x300.jpgChemprocess engineering