Key Questions to Ask When Ordering Diaphragm Valve

There are many options and several conditions that need to be considered when purchasing the right valve for a job. The more information from the field, the better the choice will be. The ultimate goal is to have the best valve for the job required at the most economical price. While it is not rocket science to do this, there is some fundamental information that needs to be taken into account to ensure the proper valve is chosen.

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5 key questions you need to ask before you get started:

1. What is the purpose of the valve and what do we want the valve to accomplish?

2. What are the required flow rates? Both minimum and maximum need to be considered.

3. What type and size pipe will the valve be located on?

4. Will the valve be used continuously, intermittent, or momentary?

5. What are the operating pressures that the valve will be subjected to?

The ultimate goal is to choose the smallest valve that can do the job required which in turn will minimize cost, making sizing a very important aspect to get right. One of the most common ways to size a valve is to use the Cv method. As a reminder, Cv is the flow across a valve when there is 1 psi pressure differential across a fully open valve. With this in mind, it’s time to start:

• Determine the maximum and minimum flows that the valve will see. This is very important as a valve working below its minimum flow requirement may hunt and fluctuate, causing pressure spikes downstream, which can lead to pipe bursts.
• Determine the pressure differential (can you get specific, tools used). A good guide is to have a minimum of 5psi between inlet and atmosphere if the valve bonnet is being vented to the air, or 10psi if the bonnet cover is connected to the downstream.
• When you have this information the most common method for calculating size is to use the Cv method, using performance curves. To measure the flow through an open valve you can use the following formula: 

                                               Q (gpm) = Cv x Square Root of pressure differential 
                                               Cv = Q/ Square Root of pressure differential

• After calculating the Cv for the application using the formula above, go to the manufacture’s catalog or data tables for the Cv valves of each size and model of valve. Always chose a valve that has at least the Cv value at the number calculated using the formula and your application requirements. It is always better to have a little safety so the CV of the valve you chose should have a higher value to give flexibility for the future if higher flows are ever desired.

Sizing Example using the Manual Cv method for a Pressure reducing valve:

We require to size a valve to handle gpm Maximum Flow, Inlet Pressure is 100psi, Outlet Pressure is to be 70psi.

i. Solving for Cv
Pressure differential = 100psi – 70psi = 30psi
Flow(Q) = gpm Solving for Cv = / √30 Cv = 639

ii. Compare Cv and Flow Frequency Capabilities for Full Port (106) and Reduce Port (206) valves:

Table 1, S106 Full Port Valve Bodies

Table 2, S206 Reduced Port Valve Bodies

iii. Valve Selection:
Cv = 639, Maximum continuous Flow = gpm

In this example the 10” 206 body is the best selection, as it meets the Cv requirement as well as the application meets the continuous flow recommendation for this valve. While the 8” S106 body meets the Cv requirement, the continuous flow recommend is below what is required for this application. Therefore if using the S106 full Port body, one would require using the 10” size as well. Full port valves cost more than a reduced port valve of the same size, therefore using the 10” 206 meets all the requirements as well is the one that is most economical.

• In any valve sizing exercise there is always an element of choice by the system designer. Is the flowrate given realistic? Are the pressures given accurate and proven? As the person responsible for sizing (while graphs, charts and even software calculators are necessary), having a clear understanding of the issues certainly makes for a more educated choice.

On this page, GEMÜ provides a brief overview of common valve types and their features.Valve, valve selection, select valve, select valves, valve type, valve feature, valve features, chemical resistance, list of compatible productsChoice of equipment for procedures,

processes and process materials

Additional reading:
The Ultimate Guide to Choosing stainless steel precision casting

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Within a plant or piping system, every process places different demands on equipment and valves. There is therefore a wide and extensive range of different designs and types available worldwide. The functionality, service life and safety of the plant, and not least the product quality produced, are therefore particularly dependent on the correct choice of valve, measurement and control components.

Analysis of the equipment requirements

Analysis of the requirements is divided into three categories: 

• Procedural and process requirements
• Media influences
• Technical plant requirements

To ensure that none of the operating parameters and requirements is neglected and that potential economical factors are not overlooked, it is very helpful to record all criteria in writing. The selection diagram can also be used to select other components such as pumps, filters, sensors, etc.

Procedural and process requirements

Category 1: Determining the process parameters

• Operating and ambient temperature
• Operating pressure and pressure rating
• Volumetric flow (Kv value) and flow velocity
• Other performance requirements, e.g. mixing, distribution, control and regulating applications.

When determining these parameters, it is important that all operating conditions are taken into account. Frequently, attention is only given to the actual process. Working situations such as cleaning and/or sterilising a plant are often overlooked. However, completely different operating conditions may well come into play, which place much greater stresses on the pipework components than the actual plant operation, and which may well have a negative impact on the function and service life.

Media influences

Category 2: Determining the media parameters

• Chemical properties (inert, corrosive, explosive)
• Mechanical properties (contaminants, particles, bubble formation, abrasion, viscosity)
• Electrical properties (conductivity, static charge)
• Aggregate state

The specific properties of the working media (fluids) must be examined with respect to all of their relevant physical and chemical properties. In addition, potential interaction, for example, between temperature, pressure or aggressivity based on concentration, should not be overlooked. Equally, the flow velocity has a direct effect on abrasion (including particle content) of the medium and/or formation of cavitation. It is always important to clarify the question: Is there only one working medium, or will the equipment be used for mixtures, compounds, cleaning agents, sterilisation media or other additives? Even the smallest addition of other substances can have a dramatic effect on the service life of the materials and the seals and gaskets.

Technical plant requirements

Category 3: Determining the existing and/or required plant design

• Required control function (manual, pneumatic/hydraulic, motorized, magnetic)
• Safety requirements (explosion protection, dangerous volatile substances, emergency function)
• Ambient conditions (cleanroom, hot/cold, dusty, vibration, chemical, damp, outdoors, saline and corrosive vapours => corrosive ambient conditions)
• Existing plant design (PLC, fieldbus/communication interfaces, control medium)
• Compliance with standards and regulatory codes

With an already existing plant or in established premises, numerous factors must be taken into account. However, also in a new building, various parameters already exist. Typical examples include control technology for installed component actuators (compressed air connections present or not) or level of plant automation (feedback/control via PLC required or not). Also mobile solutions, predominantly in water treatment, determine various parameters, for example normally only manual or motorized actuators can be used in this instance.

Equipment technology

Following precise analysis of the equipment requirements and other factors, the most suitable equipment can now be selected from an extensive product range. For this purpose, it should always be ensured that the provider also has a corresponding wide range of products and versions. If this is not the case, there is always the risk that the wrong equipment or an unsuitable device will be recommended due to limited availability. Where possible, accessories should come from the same product range. Alternatives to this are accessories that have already proven their compatibility with the same device in a plant.

Optimization of the selected equipment

Once the valve has been defined, a further step must be carried out. In addition to the "standard version", many valve manufacturers offer additional sub-versions, which offer an excellent performance profile. GEMÜ, for example, offers multiple body/seat and actuator sizes for a connection size. Among other things, this helps to prevent unwanted physical phenomena such as cavitation and to reduce operating costs. For example, the use of smaller actuator sizes based on a more cost-effective user profile can save energy during operation (keyword: Oversizing)

Solution approaches

Based on the operating parameters and the application conditions, there are normally a number of solution approaches. The technically best variant of a valve is often – in our experience – also relatively expensive. Therefore, plant engineers and operators also want to consider the "second best" type. This normally also satisfies all the requirements, but may come up against its limits when it comes to service life and functionality.

Cost benefit analysis

When selecting the latter, the "second best" solution approach, it is important at a later point in time to analyse whether, in fact, this offers the most cost effective solution. If, for example, a material is less resistant to a medium and the valve body has to be replaced at close intervals (servicing costs, installation time), it may be more cost effective to change to another, technically superior material.

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