Technical comparison between 3 major Water Control valve styles used in South Africa and Worldwide
The South African water industry uses 3 different styles of standard self-actuated pilot operated control valves, as listed below. Due to confusion caused by â€œmarketing hypeâ€ and claims by sales staff, a technical analysis is perhaps the best way to distinguish between the different models on offer in the South African market. This article only compares the standard designs on the market today, and does not include the latest new designs of axial flow patterns, such as the C-valve with all its unique benefits.
Valve 1: Cla-Val, and some lesser known copies
This is one of the originators of water control valves, and is manufactured in the USA, as well as some other minor factories around the world. It has a standard globe style body, which they have continued to use due to its beneficial design features over the Y-style valve.
Some negative features of this design
1. Main Seal Design. The main seal is a flat resilient rubber disc, which is forced onto the stainless steel body seat during closing. This causes a â€œcuttingâ€ action into the rubber seal, and can result in early wear and leakage of the seal.
2. Assembly. When the flat seal disc is fitted into the diaphragm assembly, a matched set of spacer washers are required. These have to be of the right quantity to ensure proper retention of the seal disc, and are often left out by unknowing maintenance staff. Incorrect quantity of these spacer washers also causes leakages of the valve, and leakage past the stem into the control chamber, which causes faulty operation.
3. Cavitation Resistance. The standard Cla-Val series 100 valve has a cavitation resistance which generally allows a pressure drop ratio of 3:1 without causing cavitation. In order to improve this ability, Cla-Val evolved the 600 series valve, which has a reduced seat area, and which improves the cavitation resistance to a 4:1 ratio. Caution has to be taken in that this valve has a reduced capacity of a one size smaller valve. Sales staff often take advantage of the better price of the reduced bore valve, where the client effectively receives a smaller valve with lower capacity.
Cla-Val does manufacture an anti-cavitation trim, which can achieve much better pressure drop ratios, but it comes at a steep price â€“ much higher than, for instance, installing two valves in series, or a valve with inherently higher cavitation resistance.
Valve 2: Bermad, and some lesser known copies – distinguishable by its Y-Shape body
Manufactured in Israel and originally designed for the irrigation industry.
This design was chosen for its inherently lower pressure drop, which benefits the irrigation industry where energy saving is of utmost importance. The reason for the lower pressure drop is the lower resistance inherent in the Y-Pattern body (less tortuous flow path). There is also no bottom bearing in the seat of the valve which offers some extra resistance to flow. In most control valve applications, lower pressure drop is a disadvantage, as the function of modulating control valves is to reduce energy. Control valves with an inherently low pressure drop must have accessories added to make them function properly in most applications.
Some Negative features of this design
1. Single Bearing for the Diaphragm Assembly. Although this design offers less flow resistance, it is also its single biggest weakness. When pressures (and accompanying pressure differentials) increase, the unsymmetrical flow pattern around the disc causes unequal forces and turbulences, which result in early wear of the single bearing. After a short period of operation (and depending on the severity of the flows and pressure drops), a common sign of bearing wear is a â€œrattlingâ€ sound emanating from this design. The manufacturers and marketers of this valve recognise this weakness, and will install a V-Port onto the valve disc, with reasons given for better control at lower flows. The truth, in most cases, is that the V-Port is installed to provide a second bearing in order to stabilise the diaphragm assembly at high flows, and differential pressures. However, the V-Port (which comes with extra costs), reduces capacity of the valve, and in most cases a larger valve is recommended with even higher costs.
2. V-Port. This additional accessory (mentioned above) provides the benefit of better control at low flows, leading to better stability. However, only the lower limit of stable flow is improved, whilst very low flows still result in instability â€“ so the V-Port is not a superb solution. Again, care has to be taken to ensure that the capacity of the valve is drastically reduced by the addition of a V-Port, and a larger valve may have to be selected. A further disadvantage of the V-port is that most of the energy destruction is now transferred to the smaller V-section, instead of over the complete circumference of the body seat and seal disc. This results in higher wear rates in this area, and a â€œwire drawingâ€ effect, which causes damage to the V-port with protruding â€œraggedâ€ edges. This can cause â€œjammingâ€ of the V-port inside the body seat, and total non-operation of the diaphragm assembly and valve function.
3. Cavitation Resistance.The Y-pattern design has the worst inherent cavitation resistance compared to all the globe designs, and normally around 3:1 maximum achievable pressure drop ratio. This is understandable when one considers that the flow path is the least â€œtortuousâ€ compared to the globe designs. An anti-cavitation trim does exist, but also comes at a high price.
Valve 3: Watts, and copies â€“ amongst others the Singer, GA
Industries and Ultra ACV Control Valve.
This design came about as a development of the Cla-Val, with some changes in flow pattern, robustness, and ease of maintenance. This design has been accepted by many â€œnewâ€ manufacturers as the valve of choice, as it has numerous benefits, as explained below.
Like the Watts, this valve has its diaphragm in the horizontal plane, causing the diaphragm assembly to have a straight up and down movement.
Features of Valve 3
1. Cavitation Resistance. Inherent cavitation resistance of the ultra ACV is a 4:1 pressure drop ratio. This has been achieved by increasing the height of the body above the seat area (compared to Cla-Val), and allowing most of the energy to be dissipated in an increased volume of water.
2. Increased Capacity. This is due to an increased flow path area, and evidenced by a larger body compared to Cla-Val. Comparison of CV values shows this to be approximately 10% higher capacity.
3. Quad Seal as Main Seal. This came about as a further development and improvement over the flat disc seal. Due to its smaller cross sectional area, the quad seal is protected from the flow velocity, and has proven to have a much longer life because of this.
4. WCB Steel Body and Bonnet. This material is much stronger than cast or ductile iron, which has resulted in a lighter body. Corrosion resistance of WCB is also better than ductile iron, although this does not play a major role, as the body and bonnet is coated with a high quality fusion bonded epoxy (FBE).
5. Ease of Assembly. The assembly of the internal parts consists of fewer components than both of the other makes, and is also easier, with no possibility of incorrect procedures.
Valve 3, which was originated by Watts, improved on the Cla-Val design by ensuring the body design offered higher capacities and better cavitation resistance.
In addition the quad seal offers much better life of the seal (meaning less maintenance). The valve, in addition, has all the benefits over the Y-type, as explained above.