LINEAR DISPLACEMENT VALVES
These are all valves that move perpendicularly to the pipe where they are installed in order to close.
As in everything in this life, there is the exception that proves the rule, the slanted seat valves. These valves close at 45º in relation to the pipe where they are installed, but they are also considered linear displacement valves.
We may also have noticed that all the valves in the following list are called multi-turn valves in their descriptions.
MULTI-TURN VALVES
(linear displacement)
Its definition is very simple, they are all those that in its manual valve facet we need to give multiple turns to the handwheel to open and close them.
- gate and gate gate valves
- diaphragm or diaphragm valves
- pinch valves (pinch valves)
- globe or seat valves
- globe or angle seat valves.
- globe or angle seat valves
- needle valves
- piston valves
ROTARY DISPLACEMENT VALVES
They are all those that to open or close need a 90º turn of their axis to operate them.
We may also have noted that all the valves in the following list in their descriptions were referred to as ¼ turn type valves .
- Ball or ball valves
- sector ball valves
- plug valves
- butterfly valves.
All of the above is of great importance when we want to automate a valve.
The types of actuators that we can install will be of the same electric, pneumatic and hydraulic types for linear and rotary displacement valves, but not the construction of the same, in the linear actuators we need the actuator to raise and lower the shaft and plug of the valve perpendicular to the pipe and in the rotary actuators we need the actuator to rotate the shaft and the plug 90º.
Another point of great importance is the manoeuvring time.
A globe valve is a multi-turn valve with linear displacement, this means that in order to open it, for example, we will need to turn the handwheel a few times until it is completely open.
The opening time will be slow as we need to physically turn it, this will give us a number of advantages and disadvantages.
ADVANTAGES:
- By opening slowly we avoid the so-called water hammer.
- The aperture of the shutter can be set at any point of the path getting a good regulation manually.
DISADVANTAGES:
- We cannot open in a fast way.
- We will not be able to close quickly.
A ball type valve is a ¼ turn valve with a rotary displacement, this means that to open it we only need to give a quarter turn to the lever to open it completely.
The opening time will be very fast as we only need to physically give it a little twist, this will provide us with a number of advantages and disadvantages.
ADVANTAGES:
- We can open in a fast way.
- We will be able to close quickly.
DISADVANTAGES:
- By opening quickly we do not avoid the so-called water hammer.
- The aperture of the shutter will be difficult to fix at any point and it will be difficult to get a good manual adjustment.
- The opening degree of a plug or butterfly ball valve is not proportional to the flow rate.
- This means that if we open 20% of the valve, 20% of the fluid flow will not be passing through, which makes the regulation aspect of this type of valve even more difficult.
How to avoid water hammer on ¼ turn actuated valves?
- The first tip would be to be cautious when operating them.
- Since caution is not sold as an accessory to a valve, I will explain a little trick that can avoid more than one problem.
- If we take a ball, plug or butterfly valve and we adapt a manual reducer we will get two things the first one that the valve will be more expensive and the second one that is the good one, that to operate it it becomes a multi-turn valve in its operation, eliminating the possibility of opening them in a fast way.
- This is a common practice for butterfly valves installed in e.g. fire fighting systems.
A HOMEMADE EXAMPLE: To get a real idea of what is a good regulation and a water hammer I will tell you the difference between two valves that surely we have all operated and seen how they work.
Is this the typical tap of a lifetime or a poppet or globe valve?
I think if you've been paying attention you already know the answer, plus it's one of the best valves ever invented and it's intelligent compared to a modern single lever tap.
With this type of valve I have never splashed my pants or my shirt when I went to drink or wash my hands, something I can't say about a single lever faucet that has put me in more than one situation when I have splashed myself by activating it.
The difference in reaction between the slow tap, being able to adjust the flow rate and the abruptness of a mixer or a ball valve is what we have been talking about before.
The typical rumbling of the pipes when opening a quick opening tap is what we call water hammer. This term will be technically clarified at the end of this post.
