Of the many technical aspects which face the engineering industry, understanding the forces at work inside a system is perhaps the single most important, and also one of the most complex to fully comprehend.
Within both pneumatic and hydraulic systems, valves are an integral part of the design which contributes to the efficient and safe operation of a process. Not only do they play a key part in accident management and prevention, they also heavily contribute to the efficient and smooth-running of the system. This is where a solid comprehension of valve dynamics is necessary.
What Is Meant By Valve Dynamics?
Valves are mainly regarded as an on/off service for a supply line but further to this, they are the point of control as they regulate pressure and flow rates through manipulating the flow of either a gas or liquid. They are also frequently used in a system to provide directional control, with non-return valves preventing back flow down the network.
This is achieved by adjusting the size of the aperture in the pipeline, by moving a seal in and out of position and it is this movement that is referred to as the valves dynamics.
As the movement of the valve is a fundamental element to its design, valves are broadly classified by the directional action in which they operate, and this can then be further defined through sub-classifications. The five main types of valves used in engineering today are:
- Linear
- Rotary (multi and quarter turn)
- Diaphragm
- Gate
- Pinch
Each and every type of valve is composed of two main parts, the seat which is the name given to the outer casing that attaches the valve in place and is predominantly responsible for providing a functional seal and the body which is the internal, moving mechanics of the valve that which delivers the control element. However, these are not the only considerations to account for when discussing valve dynamics.
An Introduction To Fluid Dynamics
The main goal in the field of fluid dynamics is to identify and manage the forces at work within a mass flow rather than the specific moving parts, which are used to control the flow rate.
The major effects on flow dynamics within a system occur with a loss of continuity in the mass flow, caused by interruptions and changes in the environment of which they’re being transported. For example, if a pipes cross-section area is halved, the velocity of the flow will be doubled. Both of these aspects are unavoidable with the insertion of a valve into a system.
As such, the initial selection process of choosing a control valve needs to ensure that the valve is sufficiently streamlined within the design to not cause an excessive disruption to the flow.
This not only means ensuring that the valves aperture is large enough to handle the required peak flow volume, but that the throat area and seat fittings of the valve too are sleek enough for operational purposes, as they can have a huge impact on the speed and turbulence of a flowing mass.
The Right Angles And Correct Configuration
In addition to the size and shape of the valve, there are several other key factors that need to be considered with the actual physical design of the valve itself, such as:
- Speed of opening/closing – The dynamic response rate and reaction times of the valves opening and closing under working conditions is appropriate to the systems needs.
- Quality of the seal – Minimal amount of leakage to allow for a tight control of the flow rate and/or operates within the optimal pressure range.
- Valve weight – A lower mass inertia will allow for a reduction in the required spring rate. This, in turn, makes for a more durable component as there is less strain within the entire valve train.
Perhaps the most crucial factor in achieving the required flow rate of an acute system is the seat angle and positional placement of the valve in the system. This factor alone can affect all of the various problem areas that are associated with poor valve dynamics, including:
- High-pressure spikes as valves are opened
- Low-pressure spikes upon opening valves
- Accelerated deterioration of valve components
- High noise levels produced
It may be tempting at this stage to view valve dynamics and the use of fluid dynamics as an overly large hammer being used to crack an egg, but there are greater risks involved than a simple reduction in power or loss of system efficiency.
Catastrophic Failures And How To Avoid Them
Similar to the chaos theories outlined in the butterfly effect, a moving mass of either liquid or gas can generate large forces as a side effect of smaller factors. This is most evident with problems associated with Pulsation and cavitation.
Both risks are caused through the eyelet of a valve when air pockets or vacuums within a liquid occur. These spaces can then collapse and send a huge shockwave through the system, which causes destruction to machine parts and components, generally resulting in a total system failure.
These problems can be relieved through the use of tools such as pulsation dampeners. However, it is always best practice to resolve a problem at source rather than applying a countermeasure and knowing how to limit, if not completely eradicate a problem, is always a financially sound practice in the long term.
Whilst the field of volume flow rate and system dynamics is a complex and often intimidating area of engineering to begin exploring, there is a technological aid to help guide us through this increasingly convoluted world.
Computer Modelling
Because of the number of variables and the subtleties of each, it is often too complex a problem to solve on experience and knowledge alone.
To gain a true picture of the internal workings and identify the correct solution to the needs of the design, whilst factoring many details from varying air pressures within a system to air friction and temperature based volume variations, computer models allow us a gateway into viewing this data in a more digestible format.
To assess the efficiency, safety and holistic integrity of a system, historic operational data is run through a customized computer model, of which there are now numerous options on the market from free tools to bespoke applications. This then allows an experienced user to experiment with variables in design to achieve a dynamically robust system.