Sonic Velocity 4dq59

Sonic Velocity – Piping Design Sonic velocity is also known as speed of sound. In chemical engineering, sonic velocity is defined as the sound velocity of the internal fluid. From my knowledge, this term, sonic velocity is important to if the internal fluid across the valve achieves “choked condition” or “critical condition”. When the fluid achieves that condition, it is described as choked flow. Choked flow is an undesired condition to the process flow system. It is a condition in which the mass flow rate will not increase with further decrease in the outlet pressure (P_outlet) and fixed inlet pressure (P_inlet). When choked flow is achieved, the Mach number will be equal to 1 (Ma = 1). The formula of Sonic Velocity is as defined below: (Rule of thumb) [Source: Branan, C. R. , The Process Engineer’s Pocket Handbook, Vol. 1, Gulf Publishing Co., 1976] Sonic velocity, Vs = sqrt (K*g*R*T/Mw) Where, Vs = sonic velocity with unit of ft/sec K = /Cv g = 32.2 ft/sec2 R = 1544 ft lbf °R−1 lb-mol−1 Mw = Molecular Weight T = Absolute temperature, °R Sometimes, clients request for this computation in the dynamic simulation (HYSYS). This is to ensure if the flow achieves ‘choked flow’ throughout the simulation analysis. In HYSYS software, sonic velocity is not included in the “Stream” Properties Tab. It requires doing a manual computation using the intrinsic property of the fluid. This can be done by inserting a HYSYS “Spreadsheet” icon in which the required variables (K and T) can be imported from the “Stream” “Properties” and “Condition” tabs. After that, the sonic velocity formula can be input into one of the cell in the spreadsheet using the variable imported from the “Stream” “Properties” and “Condition” tabs. When the integrator is activated and calculation integration step begins, the sonic velocity result will be automatically generated itself for every single integration time step.

In order to monitor the progress of the flow condition (choked flow or not), another flow speed is inserted next to the flow sonic flow. The flow speed can be described as v = (actual volume flow) / (internal piping cross sectional area) Sonic velocity and flow velocity are plotted in a strip chart. As the integrator is activated, the calculated result of sonic velocity and flow velocity will be plotted in the strip chart. From the strip chart, it can determine if the flow achieves the choked condition. For non-choked flow condition, the flow velocity must always be lower than the sonic velocity. A token of appreciation to Phil Leckner’s explanation: “Some people do not allow the velocity in the downstream piping to ever achieve a flow greater than 75% to 80% of sonic. You can do this by increasing the piping size but of course, you pay more for this as well. I don't necessarily subscribe to this limitation. I will size the pipe for sonic velocity but then choose the next larger standard pipe size. This will bring the velocity down somewhat. I then have the piping designers design the pipe for potential vibration if necessary.”