Valve dynamics is essentially a term to describe the valve flow and movement made by a valve to control the flow of gas through its opening.
While this itself is an easy principle to understand, the field of valve flow and dynamics is a complex and involving area which lies at the heart of a valves ability to control gas within a system. There are many subtleties and nuances involved with calculations that model the behavior of gas in motion, including factors relating to both the valve and the gas. To gain a better understanding of the dynamics involved, it’s worth reminding ourselves of the basic principles at work inside the valve.
How a valve works
From a very basic viewpoint, valves work by altering the size of an opening through which gas needs to pass to progress along its path through a system. The size of this aperture governs the amount of gas that can pass at any given time and so, regulates the total flowrate throughout the system.
While the valve trim is a term used to identify the moving parts within a valve, it’s the role of the valve disk to move into a position that permits or prevents gas flow. In addition to being a simple switch between an on and an off position, this disk can be manipulated so that the size of the aperture can be tightly controlled.
The response time of a valve is the amount of time that it takes for this manoeuvring to occur, while the speed of response is a term often used to define the whole process from identifying the need to make an adjustment, through to the completion of this change. Both of these variables are important aspects in judging the performance level of both the control valve and the system as a whole.
Valve flow
To understand what constitutes valve dynamics, you also need good knowledge regarding valve flow. The active flowrate of a gas through a control valve should be kept between practically manageable parameters, with a figure of 12 feet per second traditionally used as the maximum threshold for gas velocity. A speed in excess of this typically creates too much pressure and larger forces that can seriously impact the durability of parts within a valve.
It’s also important to note that this and all other velocities used in measuring flowrates are average figures rather than a true value. This is due to two specific reasons. First off, the naturally occurring fluctuations in speed within a mass of moving gas. Secondly, the variable speed with which a gas flows across a valves seat during a cycle of between closed to open and back to closed again.
Dynamic pressure
The viscosity of the gas being transmitted also plays a telling factor in valve flow. With different gasses possessing different characteristics, you will need to account for the effect that the specific gas being transported has on the flowrate. Often referred to as fluid power, the hydraulic fluids flow coefficient is used to provide comparative quantification between alternative fluid materials.
The flow velocity is also dependent upon the amount of pressure within the system forcing the fluid forwards. This dynamic pressure is also a variable figure that fluctuates independently across the whole system. This is due to the natural way in which molecules innately compress and extend within a liquid or gaseous state.
While this dynamic pressure is created by the momentum the natural gas carries as it progresses through the system, an initial force is required to propel the gas. This energy gradually dissipates along the course of the gases journey, through factors such as friction. However, pressure drops can also be the result of engineering and mechanical factors.
Pressure drops are caused both by the gas passing through the control valve as well as other contributing factors such as inline equipment, pipework, and environmental conditions. Calculating the size and disruption of these drops are, therefore, an important aspect in accurately modeling the actual dynamic pressure at any given point in the system. This includes directly before and after the gas passes through a control valve.
Valve flutters and imperfections
Compressor valve flutters are one of the many minor details that affect the true flow of a natural gas and by understanding such details, a clearer picture of gas dynamics can be constructed.
While the principle of a closing valve has the disk connect with the seat to form a hermetic seal, in actual practice this is not a smooth process. When the disk first makes contact with the seat, the force of both the movement and the gas acting upon the disk as it tries to force through the barrier. This causes the valve disk to reverberate before a tight seal is formed. This has the result of allowing some gas to continue passing through the control valve. Gas passes through even after the point of time in which the valve flow is technically considered to be closed.
While it is possible to design valves in such a way as to limit the amount of flutter in a valves action, it’s impossible to eradicate it at a finite level. Therefore, by restricting the amount of effect this action has and monitoring the levels in operation within a valve, it’s possible to effectively nullify the effect to a satisfactory degree.
Theory vs practical
What this means in real terms, is there’s a perceivable difference between theory and practicality. This is a major argument for the need to continuously calibrate the effectiveness of the equipment.
While the aim of valve dynamics appears to be to accurately predict the exact valve flow and flowrates of a gas moving through a control valve, much of the research in this field actually has a slightly different intention.
Modeling is used essentially as a tool to gain a better understanding of the gas flowing through a system. Because gasses don’t always act exactly as predicted, especially gaseous fluids that are comprised of molecules moving with greater energy than that of liquid molecules, this allows for a margin of error within calculations. This ultimately makes these models more accurate in the long-term.