Permanent pressure loss is an important matter of which every engineer, designer or technician should be aware. Several sources contribute to the pressure loss of a system, the most important being: pipe friction, valves and measurement devices. Whenever a new pipe section or equipment is added in the line, additional pressure loss will be created. Fluid velocity also plays an important role in pressure loss: the faster the fluid is travelling, the greater the pressure loss. This is valid for gases and steam as well as liquids, and the reason pressure loss must be taken into consideration is that it makes the pump, compressor or boiler work harder for generating the same flow in the system. In other words, more energy is needed – and energy is money.
There are many different types of flowmeter and all of them have different characteristics of permanent pressure loss. In the case of magnetic and ultrasonic flow meters, they do not create additional loss, or at least less in comparison with a straight length of pipe with the same dimension as the flowmeter. Other flowmeters have a more significant pressure loss due to their design. For example, a Coriolis flowmeter with its curved-tube; or the vortex meter and DP-flow primary elements (orifice plate, nozzle, venturi tube, v-cone, wedg-meter, Pitot tube and others) with their pipe restriction.
Pressure loss should not be confused with pressure drop. DP-flow primary elements have a pressure drop inside the meter section. The pressure measured upstream the restriction will be greater than the pressure measured just downstream the restriction. Further downstream from the primary element the pressure recovers to a level not quite as high as the upstream pressure. The difference between the upstream pressure and the recovered downstream pressure equals the permanent pressure loss.
A characteristic of some DP-flow primary elements (orifice plates, nozzle, venturi tube) is that during the engineering phase, the maximum permitted pressure loss that the primary element may create can be calculated in advance. Hence the flow measurement can be optimised to meet the maximum authorised permanent pressure loss, simply by adapting the restriction area.
For a steam application, where a very low permanent pressure loss is demanded, the use of a venturi tube is an adequate solution. Besides the possibility to optimise the restriction area, there is also the possibility to modify the diverging angle of the outlet cone between 7 and 15°. The lower the angle, the more pressure recovery is possible. The energy saving attributes of a venturi tube, justifies its use in larger pipes.
Permanent pressure loss is one of the characteristics to consider when choosing a flowmeter according to the needs of the application. The choice of the primary element may be important if pressure loss is an issue or is limited by process requirements. Reference texts show the 2 min pressure drop as a function of the differential generated as:
• Long pattern venturi tube: 10-15%.
• Short pattern venturi tube: 10-20%.
• Venturi nozzle: 10-20%.
• Nozzle: 5-60%.
• Orifice plates: 5-70%.
The guideline below can be used when selecting primary elements for steam/air applications.
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