Valve Terminals System Solutions — Hype or Prerequisite for Tangible Monetary Benefits?
Individual valves or valve terminals to control pneumatically-automated process valves? What are technical and economic advantages of these two methods? How to judge the influences with regard to commissioning? Answering this questions can result in more than 40 percent cost savings.
In the chemical industry you will find mostly individual solenoid valves for the control of pneumatically automated process valves. This kind of valves usually has a Namur interface and is direct mounted on the rotary actuator. A different way is a complete automation or system solutions which include all necessary components to control an installation from the location of the process valve up to the field bus interface. These solutions will be offered par example from Festo and include:
- Pneumatic drive with position feedback or position controller;
- Tubing and fittings;
- Air preparation (including sensors to monitor air consumption and correct operation);
- Modular valve terminal with field bus connection to superior PLC or Distributed Control Systems (DCS).
The valve terminal is the central component in this system solution. It allows decentralized automation solutions to be assembled quickly and easily. The valve terminal CPX/MPA shown in Fig. 2 is fully modular. It can control up to 64 solenoid valve coils and process digital and analogue electrical signals for communication via field bus with higher-level automation systems. It is also possible to equip the valve terminal with a PLC, which will allow installation in dedicated plant sections e.g. fermenter to be controlled independently of a DCS.
System solutions allow significant costs savings in comparison with individual-valve solutions. This applies particularly to installations with several process valves physically close to each other, for example in pharmaceutical, food and as well in chemical production. The situation could be different in the case of large installations in the petro industry, where process valves are often a long distance apart. A costs comparison will reveal the differences between valve-terminal technology and individual solenoid valve architecture.
Valve Terminal Versus Individual Solenoid Valves
Figure 1 shows the production installation in a fine chemical plant taken as the basis for the comparison. It covers an area of approx. 20 x 20 m, is 15 m high (4 levels) and produces skin-care products and detergents. The installation is controlled by around 100 valves, mainly ball valves of nominal sizes most between 25 and 50 mm. The installation is equipped with individual valves and limit-switch boxes on the drives. The calculation has been done on the basis of existing cabinets in the installation.
The solenoid valves are mounted directly on the actuator via Namur interfaces. The solenoid valves and end-position switches are connected to the field bus via remote I/O’s.
The architecture is similar with a valve terminal. The field bus node, remote I/O’s and solenoid valves form a unit as a decentral control level. When valve terminal technology is used rather than an individual-valve concept, there is, for example, no need for a binary output on the remote I/O unit to control individual valves, or any of the associated wiring, installation work, etc.
The valve terminals have been located within the installation in a way that process requirements for opening/closing times of the process valves are fulfilled.
Furthermore, the following has been assumed in the calculations:
Installation/connection costs have been assumed to be the same for the units “Individual solenoid valve with remote I/Os” and Valve terminal Type CPX/MPA.
In the case of the individual-valve solution, a multi-core cable is used to connect up the solenoid valve and sensor box. With the valve terminal the only reduction is the number of cores in the cable. The costs difference are negligibly small. The possibility of using two cables to control the individual valves has not been considered.
The calculation is based on the list prices of Festo products and standard products from other suppliers (solenoid, valves, tubing, cables, etc.). The costs include the components, the installation of lines and cables of appropriate length, and the connection costs. The installation and connection costs are those given by the end user.
The pneumatic lines are not considered. A careful local survey was conducted to determine the precise locations of the electrical cables and pneumatic lines for all the solenoid valves, which were then entered into a layout plan. This plan was then used to work out the cable and line lengths. A similar procedure was used for the upper floors.
Calculations were done for four variants, assuming the whole plant to be classified with respect to Atex as non-Ex (indoor), Ex zone 2 (indoor), Ex zone 2 (outdoor), Ex zone 1 (outdoor).
The results reveal a significant costs advantage in favour of the valve-terminal solution in all areas.
With regard to product and installation costs, the advantage is between 14 and 42 percent. An additional point for cost saving can be the pneumatic ring line (Fig. 3). The mathematical model can also be applied to installations of other sizes as a guide. This costs comparison covers only the investment costs. It makes no allowance for the further savings which can be achieved through the use of valve-terminal technology throughout the life cycle of production installations. For example, diagnostic techniques can be used to avoid downtime due to malfunctions, and any downtime which does occur can be significantly reduced through the rapid localization of fault locations.
Valve Terminals Will Be Speed Up Commissioning
Commissioning is often neglected during the planning phase. In the case of commissioning with individual valves, the system is fully wired and fitted with tubing. A compressed-air supply is then switched on and the solenoid valves are actuated via their manual overrides in order to check that the individual valves have been correctly connected.
In the installation shown, many process valves are hard to reach, frequently requiring a ladder. It is not possible to carry out on-the spot checking of allocation listings. The control cabinets are easily accessible for plant operating personnel. This means an enormous reduction in the amount of walking required during commissioning.