In the chemical and petrochemical industry, plug valve and ball valve are not interchangeable to each other.
In terms of market demand, the rate of average use demand of them is about 8 : 1, meaning, every time 8 ball valve are used, 1 plug valves will be used. If there is a higher reliability or alloy materials and better cost performance, people can use plug valve to replace ball valve, but ball valve can’t replace plug valve in the occasion where only plug valve can be applicable to.
The plug valve mainly has the following advantages:
one-piece valve body, top loading design, simple structure, convenient maintenance, no leakage, supportting higher-strength pipe system.
. The medium in chemical process has a strong corrosion resistance. In the chemical field, corroded valve body often can be seen. Therefore, the valve in the application of chemical process must have strong permeability and resistance to intergranular corrosion. The body of plug valve should be high-level casting, with casting quality of class II and precision of grade V.
Really simple internals. The plug is completely covered by bushing. Body and plug has no wear. Replacing simple and economic bushing or top sealing device can complete the maintenance and update of the valve. Plug valve can reach long-term use requirement of 300 ℃, which has considerable competitive edge compared with ball valve that belongs to this application.
Top surface and flange are all the processing surface of plug valve. Other parts are all one-time casting molding. The inside of valve body has no processing. Comparing with ball valve, it has the advantages of lower machining allowance. Especially, when the material is INCONEL, MONEL, HARTZ alloy, plug valve has the distinct advantage of price.
According to the user's demand, various designs of plug valve can achieve a specific function. Plug valves is the best one of customized products. Plug valve is easy to cater to custom design, can be made into three-way, four-way, five-way and six-way valve; double valve group or valves group; all kinds of jacket requirements, etc.. For example, the six-way distribution valve used in polyester industry is designed according to the requirements of patent owners, while the ball valve is difficult to made in line with custom design.
Now, no clearance and face sealing are still the unique features of the plug valve. Although it has the design of the clearance like ball valve’s, the clearance just can be reduced to 1 ~ 3 mm at the most, which can't achieve the application advantages of coated surface sealing. This characteristic of plug valves enable it to deal with a lot of harsh medium, such as easy crystal and viscous fluid or those medium containing catalyst particles and deposit.
Coated bushing provides the characteristics of face sealing. It can be strictly controlled in accordance with the test requirements of zero leakage of API 598 soft sealing valve seat, achieving or sealing requirements of stem of TA Luft or ISO 15848.
Comprehensive characteristics of plug valve are long service life, high reliability. It is a good choice for a lot of corrosive, abrasive and toxic fluid as well as continuously operated device. It has the application advantages and extensive applicability in the field of chemical industry.
Disadvantages are as follows:
. The design of face sealing improves the requirements for driving torque. In the case of needing be equipped with pneumatic actuators, compared with ball valve, it will cost much more. Such as PP process, main shut-off valve of it is the plug valve, rather than the ball valve, but the high frequency switching valve is ball valve. Therefore, basically, the pneumatic plug valves are applied to the key cut-off position. Plug valve is not recommended to apply it to the occasion of adjustment, since the high frequency switching is not the advantage of it. It is the cut-off valve of high reliability.
. In some applications, the incorrect choice of bushing material or improper design of the release area of bushing deformation may cause swelling for PTFE bushing materials, leading to the change of driving torque for several times.
. Though the investment of plug valve is more than ball valve, as its advantages of reliability and the total cost of life cycle are gradually accepted by users, more and more users will continue to use the plug valve.