1 May 2006
By-the-weigh flow calibration
By Keith Cheatle
Flow calibration is necessary for any transducers or measurement systems that provide information on the amount of material transferred in a given time.
These measurements usually occur in units of mass per unit time (mass flow) or volume per unit time (volumetric flow). These two types of measurements relate to one another by the density (mass per unit volume) of the flowing material.
The major factors that affect density are pressure and temperature, and these measurements are usually involved in any flow calibration work.
The majority of flowmeters in use mount on circular pipes, and calibration facilities have to be able to match the field installation of the devices.
The accuracies associated with flow calibration are not nearly as good as those associated with calibration of temperature, pressure, mass, or electrical equipment.
National standard and primary standard laboratories can typically calibrate temperature, pressure, mass, and electrical equipment with uncertainties, ranging from one to 10 parts per million.
In contrast, the uncertainties for National Institute of Standards and Technology (NIST) gas flow calibrations range from 200 to 2,000 parts per million while liquid flow calibrations are in the 800 to 1,000 parts per million range.
The equipment required for flow calibrations can be very bulky and expensive to operate.
The water-flow-calibration facility at NIST contains an 8,000 cubic ft water reservoir tank, four pumps ranging from 100 to 150 Horsepower, weigh scales up to 50,000 pounds, and 30 to 40 ft piping runs with diameters ranging from 1 to 16 in.
The reservoir tank alone could be 80 ft long, 10 ft deep, and 10 ft wide. A five-point calibration using this facility will cost approximately $5,000 and require a four-week turn around time.
These factors mean most test laboratories or manufacturing plants cannot support in-house flow calibration facilities. It also means flow equipment tends to be the least calibrated of any measurement systems.
Many flowmeters rely on having primary elements that have a specific size and/or shape to produce a signal proportional to flow. These include orifice plates, venturis, flow nozzles, vortex meters, and turbine meters. The physical dimensions of these elements will not change unless the flowing material induces wear or the installation procedure distorts or damages the element.
An orifice plate, for example, can produce a changed differential pressure if the sharp upstream edge of the machined orifice rounds out due to flow-induced wear or nicked due to hard particles in the flow stream. It can also change if the installation or excessive flow causes bending of the plate.
If dimensional values have not changed from the initial calibration, then the period between actual flow type calibrations can be longer. The user should document the dimensional measurements that occur to support the claim that no changes have occurred. The balance of the flow measurement system still has to see a calibration on a periodic basis suitable for the type of equipment.
The differential pressure transducer used with an orifice plate, venturi, or flow nozzle should undergo calibration at least once a year. The electronics used with a vortex meter or turbine meter can be calibrated by inputting an electrical signal with the same wave shape and frequency as the signal produced by the bluff body vortex sensors or turbine blades detection sensor. The dimensional measurements, differential pressure measurements, and electrical simulation measurements must use equipment with traceable calibrations.
Flow calibration equipment is available for facilities that use a large number of flow transducers or are in the business of producing flow measurement equipment. These include provers for gas and liquid applications, gravimetric fluid-flow-calibration facilities, PVTt (Pressure, Volume, Temperature, time) gas calibrators, and reference flow calibrators.
Calibrator design ensures
The weigh tank-flow calibrator is essentially a bucket on a scale with a stopwatch to time the filling of the bucket. In practice, the bucket becomes a large tank with a highly accurate weighing system, and the stopwatch becomes a sensor based timing system with a crystal-controlled clock.
The calibrator design ensures the flow is steady during the measurement period, and it maintains and measures constant temperature and pressure conditions. A data acquisition system carries out the control of the system and the measurement of test parameters.
The calibration system also incorporates large reservoir tanks, pumps, diverter valves, flow straighteners, and long, straight piping runs of various diameters. The diverter valve changes the normal flow path back to the reservoir to a flow path into the collection tank.
The flow path reverses after a specified time or after a specified quantity of fluid has accumulated in the weigh tank.
These types of calibrators are also known as gravimetric flow calibration systems. The term gravimetric refers to weight measurement determining the flow.
A static gravimetric system measures the time the diverter valve is in a position that allows flow to the collection tank and then measures the mass of the fluid in the tank after the motion of the contents has settled.
A dynamic gravimetric system measures the time for the weigh tank and its contents to change from one mass value to another mass value as the flow fills the tank.
Both International Standards Organization and American Society of Mechanical Engineers have written standards that describe the design, operation, and uncertainties of gravimetric flow calibration facilities.
Provers are the jury
Prover-type flow calibrators are available for liquid or gas applications. Some call the liquid prover a piston prover or positive displacement liquid flow calibrator.
The gas prover sometimes goes by the name bell prover. They both use the principle of transferring a measured volume of the flowing fluid through the flowmeter in a measured time.
The piston prover for liquid calibration consists of a long stainless steel cylinder that hones to a very precise inside diameter. A close fitting piston in the cylinder displaces water from the cylinder through the meter that is going through calibration.
The piston moves as a result of air pressure on the backside of the piston. Flow rates vary by adjusting the air pressure and using throttling valves in line with the meter.
A high-accuracy linear encoder measures the piston travel, which outputs a series of electrical pulses with each pulse representing a very small change in piston displacement.
Each pulse will be equivalent to a small change in liquid volume that pushes through the meter under inspection. Knowing the time between pulses gives a measure of the volumetric flow.
Pressure transducers and temperature sensors measure the fluid conditions as it flows through the meter. These calibrator signals, along with the output signal from the meter, transmit to a personal computer based data acquisition system, which uses pressure and temperature signals to calculate density and viscosity changes of flowing fluid.
Valves direct the fluid from the reservoir to the cylinder after the flow run ends.
ABOUT THE AUTHOR
Keith Cheatle ([email protected]) is a life member of ISA and a registered professional engineer in Canada. He worked 37 years in the instrumentation section of Atomic Energy of Canada Limited. He recently published Fundamentals of Test Measurement Instrumentation, ISA Press, 2006.