C H A P T E R
For smaller systems (ISO 03 and 05), mounting the valves in a stacking arrangement is now a very popular method of assembling a number of control valves (Fig. 6.4).
In this case, valves are mounted onto a sub-plate or manifold but are designed maintenance or replacement. Also, many valves (in particular solenoid directional valves) now have standardised interfaces, making valve replacement even simpler.
The original directional-valve interfaces were defined by the European committee on fluid power known as CETOP (Comité Européen des Transmissions Oléohydrauliques et Pneumatiques), and such valves are sometimes known as ‘CETOP valves’, although the original standards have now been extended and incorporated in the international standard ISO 4401. Typical examples of ISO 4401 interface layouts are illustrated in Fig. 6.3.
PIPE FITTINGS SCREWED INTO
SUB-PLATE VALVE BOLTED TO SUB-PLATE
Fig. 6.2 Gasket-mounted valve
ISO 4401-03 (CETOP 3, NFPA D03)
ISO 4401-05 (CETOP 5, NFPA D05)
ISO 4401-07 (CETOP 7, NFPA D07)
ISO 4401-08 (CETOP 8, NFPA D08) PRESSURE
TANK SERVICE
PILOT PRESSURE DRAIN
DOWEL
Fig. 6.3 ISO (CETOP) interfaces POINT OF INTEREST
The American NFPA standard is basically the same as the ISO standard for valve interfaces.
Solenoid valves are also sometimes referred to by their nominal port diameter (e.g. NG6 denotes a 6 mm port, which is equivalent to ISO 03, and NG10 denotes a 10 mm port, which is equivalent to ISO 05).
to stack one on top of the other rather than being mounted individually. Sealing between valves is again achieved by means of recessed O-rings, and the whole stack is held together and attached to the sub-plate by bolts passing through the complete stack. The directional valve normally is the top-most valve, so all the other valves use the same interface layout (port positions, etc.) as the directional valve.
Stack mounting provides a very compact and low-cost method of mounting valves together, especially for smaller valve sizes (typically up to approximately 120 L/min (32 gpm)). However, the relatively tortuous fluid pathway through the valve stack has to be taken into account in the rating of the components. For example, fluid entering the pressure (P) port of the stack has to travel upwards to the directional valve and then down again to the service port (A or B) before flowing on to the actuator.
Similarly, return fluid from the actuator has to pass up and down the stack before returning to tank, all of which adds to the internal pressure drops within the system and a consequent reduction in efficiency.
For higher flow rate systems, therefore, and in order to reduce the space requirement of pipe-mounted valves, a manifold block construction can be used, where passageways within the manifold provide the interconnections and valves are gasket mounted onto the surfaces of the manifold (Fig. 6.5).
This approach provides a very compact system with minimal potential for leakage, yet retains the ease of serviceability of the components mounted on the block. System fault-finding can sometimes be more difficult with this approach, but pressure test points can be incorporated in the manifold to minimise this disadvantage.
The other main disadvantage with manifold systems in general is the design cost of the manifold itself. Although modern CAD (computer-aided design) software has simplified the design process, there is still a significant amount of skill and work involved in designing a bespoke manifold block, which obviously increases the cost
Fig. 6.4 Stacking valves
DIRECTIONAL VALVE
OTHER CONTROL
VALVES
SUB-PLATE OR MANIFOLD
of the system. Where several or many systems are being manufactured to the same design, this initial cost can be distributed and is less of an issue, but for one-off systems the manifold design cost can be significant.
The next step on from surface mounting valves onto a manifold block is the cartridge valve system, where the manifold acts as the valve bodies as well as the interconnecting passages. This then provides an even more compact system, because much of the volume of the components is mounted within the manifold rather than on its surface.
There are two basic types of cartridge valves: screw in and slip in. Slip-in cartridge valves are also known as DIN cartridges (after the original German standard that defined their dimensions) or sometimes logic elements (as their operation is in some ways similar to electronic logic circuitry).
As their name suggests, screw-in cartridge valves are held in place within the manifold by means of a screw thread on the neck of the valve, and interconnections from one valve to another are via drilled passageways within the manifold (Fig. 6.6). Although
MANIFOLD BLOCK
Fig. 6.5 Manifold-mounted valves
SCREW-IN CARTRIDGE
VALVE
MANIFOLD BLOCK
Fig. 6.6 Screw-in cartridge valves (Image courtesy of Eaton Corp.) POINT OF INTEREST
The abbreviation SICV is usually taken to mean
‘screw-in cartridge valve’, although there is potential for confusion with the term
‘slip-in cartridge valve’.
With manifold systems it is possible to combine several different types of valve- mounting arrangements. Combinations of slip-in and screw-in cartridge valves, together with gasket- or stack-mounted valves on the same manifold block are not uncommon.