Carefully consider the isolation valve on the intake line of the hydraulic pump

In a recent hydraulic repair shop, I was asked what I think about the isolation valve on the pump suction line and whether it is necessary to use a more expensive ball valve instead of the usually cheaper butterfly valve. The root of this problem lies in the negative effects of turbulence in the pump suction line. The argument for using a ball valve as an isolation valve for the intake pipe is that when it is opened, the full bore of the valve allows oil to flow. Therefore, if you install a 2-inch ball valve in a 2-inch intake line, when the valve opens, it will be as if it does not exist at all (at least from an oil point of view).
On the other hand, butterfly valves are not full bore. Even when fully opened, the butterfly remains in the hole and exhibits partial restrictions of irregular shapes. This causes turbulence, which causes dissolved air to flow out of the solution in the intake pipe. If this happens, these bubbles will burst when pressure is applied to the pump outlet. In other words, butterfly valves may cause cavitation.
So which is best: ball valve or butterfly valve? Well, like many problems in hydraulic systems, it depends. In a perfect world, I would always choose ball valves before butterfly valves. For intake pipes up to 3 inches in diameter, there is almost no cost loss to do so.
However, when you enter 4 inch, 6 inch, and 8 inch diameter, ball valves are very expensive compared to butterfly valves. They also take up more space, especially in overall length. Therefore, for example, in mobile applications, not only the cost of a large-caliber ball valve may be too high, but there may not be enough space between the tank outlet and the pump inlet to install it.
There is a third option. Many people mistakenly believe that the intake pipe isolation valve is essential, but in fact it is not, but there are only a few exceptions.
The first question that pops up in response to this problem is how to replace the pump if there is no isolation valve on the intake line. There are two answers to this. First, if the pump fails catastrophically and you are doing the “right” thing, you should use a filter cart to draw the oil from the tank and put it in a clean bucket or other suitable container. The fuel tank should then be cleaned thoroughly, the pump should be replaced, and the filter cart should be used to pump the oil (assuming it is still available) back to the tank.
The general objection to this is: “Oh, we don’t have time to do this!” or “We don’t have 10, 20, or many clean drums.” For those who don’t want to get the job done correctly, one solution is Seal all the permeable parts in the tank top space, and connect the industrial vacuum cleaner to the permeable part of the tank respirator. Turn on the vacuum cleaner when replacing the pump, and then repeat the exercise when the debris from the last pump failure caused the replacement pump to fail.
Of course, there are exceptions. For example, if there are multiple pumps sucking in from the same tank, or pumping 3,000 gallons of oil from the tank is impractical. Sometimes the intake pipe isolation valve is necessary. If this is the case, it is wise to ensure that they have proximity switches to prevent the pump from starting when the valve is closed.
If possible, my preferred method is to install neither a ball valve nor a butterfly valve. If you must have one, if cost or space is not an issue, use a ball valve. However, if there is a problem with any of them, then the butterfly valve is the only option.
In many applications, butterfly valves are used as pump inlet isolation valves. Large hydraulic excavators are a common example. They have multiple pumps to suck out of the large tank through a large diameter intake pipe, and there is not much space-all the components exclude the more preferred option (no valve or ball valve).
I don’t remember ever seeing a pump on a large hydraulic excavator without at least some cavitation damage, which can be considered normal wear in this application. Is this cavitation damage due to the turbulence caused by the butterfly valve? Of course it can, but many other things can also cause it. The only sure way is to compare two pumps operating under the same conditions-one with a butterfly valve and the other without a butterfly valve.
Brendan Casey has more than 20 years of experience in the maintenance, repair and overhaul of mobile and industrial equipment. More information about reducing operating costs and increasing…


Post time: Jul-08-2021

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