Choosing the best valve for bulk handling processes
Whether you are moving cement, sugar, talc or other dry products the valve selection can be a very important decision affecting your process down time, maintenance costs and employee safety.
There are a number of valve choices for both dense phase and bulk handling. Some of the potential selections are hammer valves, knife gate valves, butterfly valves, switching valves and pinch valves. When dealing with dry products we feel the best alternative is a high performance pinch valve. With all of the other valve alternatives the biggest problem is the other valves either have areas for material accumulation or have parts in the flow stream. At Larox we manufacture knife gate valves as well and in certain instances we will suggest the use of a knife gate instead of a pinch valve. However the pinch valve in almost all cases is the best solution for dry materials.
In many plants the valves at the top of the silos have traditionally been hammer valves, knife gate valves or switching valves. Hammer valves are typically labor intensive and not highly reliable. Knife gate and switching valves have a tight seal when new but typically rapidly deteriorate over time and then begin to leak. Tight shut-off is an absolute necessity when different materials are being transported to various silos. A leaking valve can contaminate a silo at a cost of $250K depending on silo size and cost of the product contained in the silo.
A pinch valve may be a better alternative for silo isolation. The greatest advantage of a pinch valve is that the rubber sleeve is built for extreme abrasion and will always guarantee 100% tight shut-off. Even when the sleeve begins to fail the typical failure is on the downstream side of the pinching area of the sleeve. So even if the valve sleeve has a hole in the downstream side after the pinching mechanism the valve will still shut-off 100% and not cause silo contamination.
Seen above (Fig. 1) are two Larox valves pinch installed just downstream of switching valves. The internals of the switching valves have been removed and all external holes have been welded shut. The plant is now using the switching valves as a Y and the Larox pinch valves are the main isolation valves to the silo.
Knife gate valves and switching valves typically become more difficult to operate over time as the internal metal components begin to wear. In particular switching valves are difficult and costly to automate. In one plant the manager of maintenance complained that the manual switching valves had been the cause of lost time injury due to the difficult operation of switching the flow from one silo to another. A 10” switching valve typically needs to be repaired one time per year. The cost according to one plant was approximately $13,000 for repair job per valve. In the picture above the cost of two Larox pinch valves was less than the repair cost of the switching valves. Also on average the Larox valves in silo isolation on average need to be repaired only once every three years. With pinch valves the only part that comes in contact with the pneumatically conveyed material is the rubber sleeve which can be replaced quickly and inexpensively compared to the repair costs of knife gates and switching valves.
In the case of knife gates the knife when opened is extracted from the flow and is subject to the external environmental conditions. If for instance the weather is raining or snowing the knife blade collects moisture and then when closed is pushed back into the pipeline. When the valve is new the seals in the packing area most likely do a fairly decent job of wiping the moisture away before it is subjected to the dry cement being conveyed through the piping system. As the valve gate packing wears the moisture is not wiped away and then pushed back into the piping system where the moist knife is coated with dry cement creating cured cement build-up on the gate. This makes operation very difficult and also further deteriorates the knife packing.
Not all pinch valves are made the same quality so please, buyer beware. The rubber quality and the valve construction play a vital role in the valve’s survivability. Larox utilizes specially cured high temperature styrene butadiene rubber (SBRT) to survive the harsh conditions found in bulk handling. Also the stem mechanism or screw is fully enclosed in a stainless steel enclosure with a grease nipple at the top of the hand wheel. The stainless steel screw mechanism is fully sealed and packed with grease for very easy operation of the valve. In most cases the person operating the valve can open and close the valve with very modest hand force. Most other pinch valves do not have this stem protection and often times the screw mechanism is left open to the atmosphere causing very difficult operation as the valve ages.
Pinch valves are the most versatile with regards to means of operation. Pinch valves can be manual, pneumatic, hydraulic or electric operation. In many cold weather locations pneumatic operated valves are avoided due to the possibility of freezing air lines. A more suitable alternative would be electric actuated valves with internal space heaters that can be operated from the control room rather than having a person travel to the top of silo to open and close valves.
The above photo (Fig. 2) is a 500mm (20”) valve and is a good example of pinch valves rubber quality and valve construction are vitally important to the valve’s ability to survive over time. The Larox enclosed pneumatic actuated valve on pneumatically conveyed dry powder. This customer used another pinch valve in this application. The other company’s pinch valve was a non-actuated pinch valve. What I mean by that is that the valve was designed to use air pressure pumped into the body of the valve housing to compress the rubber pinch valve sleeve. In these types non-actuated pinch valves the rubber cannot have strong reinforcement so that it can remain flexible. Unfortunately the non-actuated valve’s rubber sleeve failed as quickly as 4 weeks. The cement company purchased two Larox pinch valves to replace non-actuated type of pinch valves and the sleeve life in the Larox pinch valve has never failed and has been in operation more than 5 years.
In a dense phase or blow pot area there is a significant challenge to find a valve that will work for any significant time. In this area the blow pot is filled with the bulk material and then the valves on the vessel are closed. Then an air or nitrogen feed valve is opened and the blow pot is pressurized to approximately 70 psig. Then typically a 6” or 8” valve is opened and the bulk material is blown to silos or other parts of a process. After the bulk material is discharged from the blow pot or dense phase system, then a 4” or 6” valve are opened to vent the remaining pressure from the system. Then the entire process is repeated over again and again. The dense phase system usually discharges approximately every 3 minutes. So the valves in this area see extremely harsh and high cycle conditions. Many plants try numerous types of valves here. The unfortunately reality is nothing is going to survive this abuse for a very long time. In one plant they complained they were changing their existing 4” pinch valves on the vent line once per month. Larox offered a valve on trial and the valve lasted 7 times longer than a conventional pinch valve and 14 times longer than butterfly valves in this area. This was obviously significantly better than their other valves had been performing. The plant then replaced their existing 6” main bulk material discharge valve with a Larox valve. With pinch valves you can simply change the rubber sleeve and put your valve back in service. They are significantly more expensive but in time you save money on maintenance costs and lost production. So the total cost of ownership is less for high quality pinch valves than for even very inexpensive butterfly valves or poorly manufactured pinch valves.
In conclusion the valve type with the lowest cost of ownership in bulk materials, dense phase and silo isolation are Larox high performance pinch valves. The frequency of valve repairs will be much lower for high performance pinch valves than for switching valves, knife gate valves, butterfly valves and hammer valves. But by far the greatest benefit of Larox high performance pinch valves is the elimination of contaminated processes, lower total cost of ownership and reliability.
10” pinch valves (Fig. 3) utilized on silos where they only utilize the valves for ship unloading. These particular valves replaced knife gate valves where the knife gate had been more than grooved and destroyed by the pneumatically conveyed bulk material.
Choosing the Best Valve
Choosing the best valve for bulk handling processes
Whether you are moving cement, sugar, talc or other dry products the valve selection can be a very important decision affecting your process down time, maintenance costs and employee safety.
There are a number of valve choices for both dense phase and bulk handling. Some of the potential selections are hammer valves, knife gate valves, butterfly valves, switching valves and pinch valves. When dealing with dry products we feel the best alternative is a high performance pinch valve. With all of the other valve alternatives the biggest problem is the other valves either have areas for material accumulation or have parts in the flow stream. At Larox we manufacture knife gate valves as well and in certain instances we will suggest the use of a knife gate instead of a pinch valve. However the pinch valve in almost all cases is the best solution for dry materials.
In many plants the valves at the top of the silos have traditionally been hammer valves, knife gate valves or switching valves. Hammer valves are typically labor intensive and not highly reliable. Knife gate and switching valves have a tight seal when new but typically rapidly deteriorate over time and then begin to leak. Tight shut-off is an absolute necessity when different materials are being transported to various silos. A leaking valve can contaminate a silo at a cost of $250K depending on silo size and cost of the product contained in the silo.
A pinch valve may be a better alternative for silo isolation. The greatest advantage of a pinch valve is that the rubber sleeve is built for extreme abrasion and will always guarantee 100% tight shut-off. Even when the sleeve begins to fail the typical failure is on the downstream side of the pinching area of the sleeve. So even if the valve sleeve has a hole in the downstream side after the pinching mechanism the valve will still shut-off 100% and not cause silo contamination.
Seen above (Fig. 1) are two Larox valves pinch installed just downstream of switching valves. The internals of the switching valves have been removed and all external holes have been welded shut. The plant is now using the switching valves as a Y and the Larox pinch valves are the main isolation valves to the silo.
Knife gate valves and switching valves typically become more difficult to operate over time as the internal metal components begin to wear. In particular switching valves are difficult and costly to automate. In one plant the manager of maintenance complained that the manual switching valves had been the cause of lost time injury due to the difficult operation of switching the flow from one silo to another. A 10” switching valve typically needs to be repaired one time per year. The cost according to one plant was approximately $13,000 for repair job per valve. In the picture above the cost of two Larox pinch valves was less than the repair cost of the switching valves. Also on average the Larox valves in silo isolation on average need to be repaired only once every three years. With pinch valves the only part that comes in contact with the pneumatically conveyed material is the rubber sleeve which can be replaced quickly and inexpensively compared to the repair costs of knife gates and switching valves.
In the case of knife gates the knife when opened is extracted from the flow and is subject to the external environmental conditions. If for instance the weather is raining or snowing the knife blade collects moisture and then when closed is pushed back into the pipeline. When the valve is new the seals in the packing area most likely do a fairly decent job of wiping the moisture away before it is subjected to the dry cement being conveyed through the piping system. As the valve gate packing wears the moisture is not wiped away and then pushed back into the piping system where the moist knife is coated with dry cement creating cured cement build-up on the gate. This makes operation very difficult and also further deteriorates the knife packing.
Not all pinch valves are made the same quality so please, buyer beware. The rubber quality and the valve construction play a vital role in the valve’s survivability. Larox utilizes specially cured high temperature styrene butadiene rubber (SBRT) to survive the harsh conditions found in bulk handling. Also the stem mechanism or screw is fully enclosed in a stainless steel enclosure with a grease nipple at the top of the hand wheel. The stainless steel screw mechanism is fully sealed and packed with grease for very easy operation of the valve. In most cases the person operating the valve can open and close the valve with very modest hand force. Most other pinch valves do not have this stem protection and often times the screw mechanism is left open to the atmosphere causing very difficult operation as the valve ages.
Pinch valves are the most versatile with regards to means of operation. Pinch valves can be manual, pneumatic, hydraulic or electric operation. In many cold weather locations pneumatic operated valves are avoided due to the possibility of freezing air lines. A more suitable alternative would be electric actuated valves with internal space heaters that can be operated from the control room rather than having a person travel to the top of silo to open and close valves.
The above photo (Fig. 2) is a 500mm (20”) valve and is a good example of pinch valves rubber quality and valve construction are vitally important to the valve’s ability to survive over time. The Larox enclosed pneumatic actuated valve on pneumatically conveyed dry powder. This customer used another pinch valve in this application. The other company’s pinch valve was a non-actuated pinch valve. What I mean by that is that the valve was designed to use air pressure pumped into the body of the valve housing to compress the rubber pinch valve sleeve. In these types non-actuated pinch valves the rubber cannot have strong reinforcement so that it can remain flexible. Unfortunately the non-actuated valve’s rubber sleeve failed as quickly as 4 weeks. The cement company purchased two Larox pinch valves to replace non-actuated type of pinch valves and the sleeve life in the Larox pinch valve has never failed and has been in operation more than 5 years.
In a dense phase or blow pot area there is a significant challenge to find a valve that will work for any significant time. In this area the blow pot is filled with the bulk material and then the valves on the vessel are closed. Then an air or nitrogen feed valve is opened and the blow pot is pressurized to approximately 70 psig. Then typically a 6” or 8” valve is opened and the bulk material is blown to silos or other parts of a process. After the bulk material is discharged from the blow pot or dense phase system, then a 4” or 6” valve are opened to vent the remaining pressure from the system. Then the entire process is repeated over again and again. The dense phase system usually discharges approximately every 3 minutes. So the valves in this area see extremely harsh and high cycle conditions. Many plants try numerous types of valves here. The unfortunately reality is nothing is going to survive this abuse for a very long time. In one plant they complained they were changing their existing 4” pinch valves on the vent line once per month. Larox offered a valve on trial and the valve lasted 7 times longer than a conventional pinch valve and 14 times longer than butterfly valves in this area. This was obviously significantly better than their other valves had been performing. The plant then replaced their existing 6” main bulk material discharge valve with a Larox valve. With pinch valves you can simply change the rubber sleeve and put your valve back in service. They are significantly more expensive but in time you save money on maintenance costs and lost production. So the total cost of ownership is less for high quality pinch valves than for even very inexpensive butterfly valves or poorly manufactured pinch valves.
In conclusion the valve type with the lowest cost of ownership in bulk materials, dense phase and silo isolation are Larox high performance pinch valves. The frequency of valve repairs will be much lower for high performance pinch valves than for switching valves, knife gate valves, butterfly valves and hammer valves. But by far the greatest benefit of Larox high performance pinch valves is the elimination of contaminated processes, lower total cost of ownership and reliability.
10” pinch valves (Fig. 3) utilized on silos where they only utilize the valves for ship unloading. These particular valves replaced knife gate valves where the knife gate had been more than grooved and destroyed by the pneumatically conveyed bulk material.
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