Design of orifice plates and throttles

Orifice plates (also: perforated discs/throttle discs) as well as throttles are also particularly used to limit mass flows.

Orifice plates are often used in supply lines that carry liquids or gases from supply networks to individual parts of the plant. In the event of a control valve failure, these limit the mass flow that can be supplied to the dischargeable mass flow via the safety valve installed there. In this case, the orifice plate is safety-relevant and must be marked as such and undergo a test cycle.

Orifice plates are also frequently used in heating steam supply lines to vessel shells in order to limit the maximum heating steam mass flow into the shell as well as the maximum heating output into the vessel interior. If it is necessary to limit the heating output to ensure that the safety valve on the inside of the vessel is sufficiently dimensioned for the “vapour pressure increase of solvents” design case, the orifice plate also becomes a safety-relevant component.

Determining the mass flow via an orifice plate is not a trivial matter, especially in the case of compressible media. The mass flow through an orifice plate depends not only on the pressures and the diameter of the orifice plate but also on the shape of the orifice plate, the ratio of the orifice plate diameter to the internal pipeline diameter, and the geometry of the line. Sharp-edged orifices should be used where possible to limit the mass flow, since the formation of the vena contracta is most pronounced in critical flows and the mass flow is significantly more limited, especially at sonic velocity, than via an ideal nozzle. An orifice plate therefore cannot simply be calculated using the equation for nozzle flow and constant discharge coefficient.

The primary advantages of orifice plates over throttles are their ability to be simply and inexpensively retrofitted and their small space requirements as well as the fact that they can be quickly and easily replaced. Disadvantages include the fact that the diameter of an orifice plate can change due to corrosion or erosion and that, unlike a throttle, it is not visible from the outside. The flow through a throttle is also usually much quieter than through an orifice plate.

The optimum orifice diameter often must be determined iteratively, because for the process at lower upstream pressure, a certain mass flow is required for normal operation, which should at least flow through the orifice, while on the other hand, the maximum mass flow at increased upstream pressure must not be greater than that which can be discharged through the safety valve.

With our FLOMASTER® flow simulation software, we determine the optimum orifice diameter for your process and document it in a short tabular report. We can obtain the substance data required for calculation of most common gases from our extensive substance database.