There are many different types of fittings and valves for various liquids, but not every type is equally suitable for transporting corrosive liquids. Globally, about 10 specific types of appendages / shutters are specifically eligible for this. A short discussion of the most important valves can be found under this link.
To choose the right valve, you need to go through some essential steps. These steps are:
Check the medium
• Chemical resistance
• Pressure and temperature
• Purity of the medium with respect to the presence of particles and crystal formation
Determine which functionality (s) your valve should meet
• Adjustability or simply ON / OFF
Position indication required or not
• Full pass or not
• Leak resistance under vacuum
• Triggering of pressure surges / water stroke
Connection techniques for valves and appendages
Valve automation and appendage
Step 1: chemical resistance
To investigate the required chemical resistance of the material of the valve and associated seals, we initially follow the same logic as that for the choice of pipe materials. You can read about this in this article on the selection of corrosive liquids.
When choosing valves and appendages, however, a little more attention must be paid to the type of seals. Since valves and appendages are also dynamically and mechanically loaded with certain seals (via movements, deformations, friction, etc.), the choice of sealing material can vary. For example: the diaphragm of a diaphragm valve undergoes a very different mechanical load than the O-rings on the shaft of a ball valve. In the first case, the diaphragm undergoes a direct deformation due to the vertical movement of the shaft to which it is connected, and when closed, this diaphragm is also partially compressed against the seat of the valve housing. The O-rings on the shaft of a ball valve undergo a very different mechanical load. In this case, the movement is rotational in nature and causes friction. In both cases, it is especially important to know that valves are designed to perform a theoretical minimum number of unlocking cycles according to DIN. It goes without saying that the action of certain chemicals can adversely affect the service life and the number of available unlock cycles. However, this situation can be assessed on a case-by-case basis, preferably in close consultation with the manufacturer.
Pressure and temperature
As with tubular materials, the P / T (pressure-temperature) diagram of the valve material should be examined. Here, too, the link with the chemical resistance must be made and both aspects as a whole must be assessed. For this, we refer to the article on reading a P / T diagram.
In the case of valves and appendages, however, an additional aspect must be taken into account, namely the fact that there are often deviating P / T diagrams for valves with regard to the P / T diagrams of the pipe material. The reason for this can be found in the fact that, in contrast to pipe materials, valves and fittings are composite products, the various components of which must be able to function as a whole in all the P / T and chemical conditions provided for this purpose. This also means that in a pipe system with valves, it is not sufficient to assess only the pipe material in terms of chemical resistance and P / T, but the whole of pipe, fittings, seals, valves, appendages, etc. must be assessed, including the considered connection technology
Below you will find a concrete example of the P / T diagrams of respectively: GF PVC-U pipe material, GF membrane valve in PVC-U, GF ball valve 546Pro in PVC-U.
Steps 1C, 1D and 1E
In these steps, we discover that the specific nature of the medium can also greatly influence our valve selection. A very concrete and practically common example is found in liquids with solid particles (e.g. chemical waste water with small metallic particles from a surface treatment process) and liquids which have crystallizing properties (e.g. NaOH). In this case, ball valves are generally moderate to unsuitable, while diaphragm valves are rather suitable. It should be noted, however, that we must take into account both the nature of the particles, the size of the particle size and the concentration of particles. The table below gives a first general overview of the general possibilities. It is certainly advisable, in case of doubt, to consult the manufacturer.
Step 2: The functional properties of the valve
When choosing the right valve, it is important to know what the valve will be specifically used for. At the bottom you will find a concise overview of some common functional properties, which are best checked.
We highlight one example, viz. the adjustability. Most valves in a piping system are used either to open and close (On / Off operation) or to adjust a flow rate (modulating or adjusting operation). In On / Off operation, the focus on the use of the valve is mainly on its primary function; nl. safely open and close the pipe. In the closed state, it is essential that the valve, regardless of the medium pressure, closes as well as possible and internal leaks are avoided. In the open state, we prefer to have as full a passage as possible and thus as few charge losses (pressure losses) as possible. If we take these parameters with us and test them on, for example, ball valves and diaphragm valves, we find that ball valves mainly excel in an On / Off operation, while diaphragm valves have a much better adjustable operation. The answer regarding the controllable operation can be found in the control characteristics of the valves. It is important here that the linear and proportional zone of the control curve is as wide as possible and preferably lies fairly centrally in the diagram. This figure is clearly much stronger present at the diaphragm valves than at the ball valves. With the ball valves we see that the first 70% opening only has an effect on the first 30% of maximum possible flow through the valve. Knowing that a ball valve generally only rotates 90 ° from fully closed to fully opened, this therefore causes a very difficult adjustability / adjustability of the valve. With the membrane valves, this situation is completely different. Here we see that there is a quasi-completely linear and proportional relationship between degree of opening and flow rate between 20% opening and 80% opening of the valve. In addition, this linear zone is nicely central in the curve. An additional advantage in this connection is the fact that with a diaphragm valve you have to turn the spindle 360 ° several times, in order to get from fully closed to fully open position. This thus generates the possibility of setting a desired flow rate quite fine.
It should be noted that there are also ball valves which have a proportional relationship over a large part of their control curve. The ball in question then no longer contains a classic central bore, but rather a specially shaped recess. It should be noted here that this type of ball valves is less favorable in terms of load losses and potentially maximum flow rate and also that the proportional and linear relationship, depending on the nominal diameter of the valve, only starts from a certain opening angle. The choice between a diaphragm valve or “linear” ball valve depends, with regard to controllability / modulating character, on various parameters with regard to the process in which the valve functions. This topic is covered in more detail in… (the topic of automatic shutters).
However, when it comes to the charge losses caused on the one hand and the maximum possible flow rate on the other hand when the valve is open, we find that the ball valves, which enjoy a full passage at 100% opening, score best compared to the diaphragm valves. The tables with regard to the Kv100 values (these describe the flow rate at 100% opening of the valve and a differential pressure of 1 bar over the valve) show the differences regarding the generatable flow rate.