Valves

There are valves for every conceivable purpose in fluid flow. Ultimately they all depend on creating a restriction in which kinetic energy is created and subsequently lost into the molecular structure of the flowing fluid. Generally no special arrangements are needed to “lose” the kinetic energy but, for some hydroelectric applications, the energy to be dissipated is so great that valves have to be designed to dissipate it in a controlled manner. Where a valve controls the flow in a long pipe the inertia of the long column of fluid flowing along the pipe can lead to destructive pressures if the flow is stopped suddenly and great care has to be taken to control these pressures.

 

Figures 8-5, 6 and 7 seven show the principles of the three most commonly used valves. I have omitted all the mechanical details that are required to make these into practical devices. Figure 8-5 is a gate valve and is really the adaptation to a circular pipe of the square paddle used in lock gates. The gate is circular, has a small taper from top to bottom to fit snugly into faced matching seats. The hole through which the fluid flows is a crescent and there is a complex relationship between the area of the hole and the position of the disc. However, when it is fully open it offers little resistance to flow. It is often used in as an isolating valve in water mains.

 

Figure 8-6 is the common globe valve used where the rate of flow is to be controlled. The restriction is between the disc-shaped jumper and the seat. Clearly the path through the valve is never unrestricted and it will always offer a resistance to flow even when fully open. The valve can be adapted by reducing the size of the hole in the seat and changing the disc for a tapered needle to give a very sensitive valve.

 

The plug valve is used very extensively for control of the flow of gas. It is simple and has a good seal. The blending of the circular pipe into the rectangular hole in the plug reduces the loss when the valve is open and is necessary anyway. It can be closed very quickly and needs to be used with care.

 

The variations on these basic designs are endless.

 

The valve is not a device that works in isolation and often it is either somewhere along or at the end of a pipe. An important pipe system is illustrated in figure 8-6. It is just a pipe supplied with water from a tank and fitted with a valve at its end. That valve is there to control the rate of flow from the pipe. Usually the valve is screw operated but it may be a plug valve. As explained above whatever type of valve is used it works by creating a hole of adjustable size through which the water flows. As the area at the restriction is reduced the speed and hence the kinetic energy of the water leaving the restriction, increases from that in the supply pipe and this kinetic energy comes from the energy head on the system. As a result the energy available to overcome friction falls and the speed of the flow in the pipe falls, As the valve is progressively closed there is a switch from all the energy going to overcome friction in the pipe to all the energy going to produce a small jet of water moving at high speed with almost no speed in the pipe. There must be a relationship between the area of the restriction and the flow. A glance at the three valves shows that the flows are complicated and different for each one. This is too complex for analysis. But, if minor losses are ignored, and we say that, at any valve position, the total head on the system is used either in overcoming friction or in creating the kinetic energy of the water flowing through the restriction a useful relationship is possible. It seemed to me that the most likely way to make progress is to define an area ratio A for the partly open valve. A is the ratio of the area at the restriction divided by the area of the pipe. Then the velocity at the restriction  where  is the velocity in the pipe.

Then                        

From this                   and

                   

This can now be converted to a graph of flow versus area ratio using Mathcad.

I had never seen graph 8-4 before I plotted it for this text and at first I was surprised by its shape. I plotted initially first for one length of pipe actually 70 metres and then realised that I needed a family of graphs for several lengths. Clearly all this family of graphs are tangential to one line and that line is for no pipe at all.

 

The line in black shows how the flow changes with valve opening when there is no pipe and it is what one might expect for a variable orifice attached to the tank. But, once a pipe exists between the tank and the nozzle, the growth of the friction loss with flow starts to reduce the energy available to create kinetic energy to give this characteristic shape.

 

The valve has no characteristic behaviour as such, its behaviour is closely linked to the pipe system in which it is used. You can check this graph by using your own outside tap. All that matters takes place in the first turn.