Flowmeters must be installed properly if accuracy and repeatability of measurements are to be sustained over long periods of time. The system in which the meters will operate must have provisions for controlling flow, transitional, and no flow conditions. Both hydrostatic and hydrodynamic pressure must be an integral part of the variables controlled in the flowing system. The system design must have the ability to eliminate transient pressure conditions, eliminate thermally induced pressure increases, and exclude air or vapor.
The entire system must be kept full of product at all times. The photograph below is a typical fuel delivery gantry designed for loading rail tank wagons. This particular gantry has provisions for loading 152 rail tank wagons at one time. Four different fuels are supplied to the gantry through 16-inch pipe lines, which are approximately ½ mile long. The pumps are centrifugal type and are located between 100 and 300 yards from the storage tanks.
The next photograph shows a typical gantry position. From left to right, there is a supply line attached to an isolation valve, a second isolation valve and piping leading vertically to the loading arm. The meter is equipped with a dual channel, quadrature-pulse transmitter connected directly to a batch controller. The batch controller provides preset batch capability, flow rate control, and slow flow start and stop. There is one batch controller for each metering position.
Design For Calibration
It's essential to calibrate meters in the field. Calibration confirms that the meter is capable of attaining the required accuracy, linearity, and repeatability.
Typically, the meter characteristics are:
Mechanical Registration•Repeatability – Capable of .02% or better at any flow rate over entire range •Linearity – ± .125% over a 5:1 range •Linearity – ± .22% over a 10:1 range •Linearity – ± .5% over a 40:1 range |
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Electronic Registration
•Repeatability - Capable of .02% or better at any flow rate over entire range
•Linearity – ± .1% over a 5:1 range
•Linearity – ± .1% over a 10:1 range
•Linearity – ± .15% over a 40:1 range.
The meters are normally tested against a master meter that has been proven against a Weights & Measures certified, volumetric prover.
The figures below show both a proper meter installation and mobile master prover. The prover connections are at the right. The two valves to the left of the meter are for connecting the volumetric prover or master meter to be used for calibration. The valve mounted in the vertical line between the two horizontally mounted valves is a double-block, bleed valve that ensures there is no leakage through the valve during calibration.
Design for Standard Mounting Arrangements (straight through flow)
Regardless of meter mounting configuration, accessories such as the air/vapor eliminator must always be mounted in a vertical orientation to permit proper operation of the float-actuated apparatus. |
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Design for Special Mounting Arrangements
Special flange arrangements are available as shown. Contact the factory for details or additional information.
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Design for Hot Oil/Steam Jacket Options
Heating a meter can be accomplished by hot oil or steam-jacketing the meter rear cover and/or packing gland boss. A heat-jacketed meter does not preclude the necessity for insulating the meter and surrounding piping. |
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Design for Ensuring the System Remains Full of Product
The first consideration is filling the piping for the first time, which must occur slowly to avoid pressure “spikes” or transients caused by rapid acceleration or deceleration of the liquid. Under gravity flow conditions, it's possible to encounter pressure transients that are orders of magnitude larger than the available head pressure.
During this time, air must be removed from the system. Do this by ensuring that a positive-head condition is always present in the system (this should be designed into the system). Air or vapor in the system contributes to error in measurements while metering. After removing the air from the system, ensure that the system design does not permit air to re-enter the system once it is full of product.
Design for Thermal Shock or Expansion
Do not neglect to account for temperature considerations in system design. Take care to design the system so that there are no zones where fluid will become trapped or locked between two closed valves. For every 1°F increase in temperature, pressure in the trapped zone will increase by an larger amount that varies with the pressure of the product in the system (70 psi for LPG). A few degrees increase in ambient temperature may create a serious problem if there is no thermal expansion pressure relief in any region that contains trapped product. Typically, the pressure setting in a thermal relief valve is set approximately 10% higher than the nominal operating pressure.
Design for Proper Flow Control
The example configuration shown in the figure at the beginning of this topic shows 152 meters that monitor the delivery of four different products. 38 meters deliver HSD. Assume that one pump is selected to supply product to the meters. Each meter measures 375 gallons per minute (GPM). The total flow capacity of the pump must be:
38 X 375 GPM or 14,250 GPM.
Once the system is in operation, product flows through the first meter when the control valve opens. If the flow rate is not regulated, 14,250 GPM attempts to flow through the meter. The resulting overspeed will immediately damage the meter. It is critically important that each meter location contains a digital or hydraulic flow rate controller. These valves must be downstream of the meter to ensure that the meter remains full of product at all times.
It's important to ensure full control when starting or stopping flow. Proper control will minimize or eliminate transient pressure conditions brought on by rapid changes of velocity in the system.
Commissioning
After the metering system is installed, it is ready for commissioning. Filling the system the first time requires care and time. Ensure that the isolation valve on the inlet side of the meter is closed. Pressure the header by starting the pump. At the meter position to be commissioned, “crack” (slightly open) the inlet isolation valve until flow into the meter can be heard. Allow the inlet side to remain in this slightly opened position until flow stops. At that time, “crack” or loosen the pipe plug on top of the meter case to allow air to bleed out. Once the air is eliminated and product is present, fully tighten the plug.
Open the inlet side isolation valve completely. The system must be kept full of fluid to avoid damage to the meter. If air is allowed inside the metering element, significant damage can occur as this air expands or compresses due to system flow and pressure conditions.
In extreme cases, the presence of air can cause catastrophic failure of the measuring element because liquid velocities and transient pressure spikes may bend or break critical measuring elements. It is critically important to control the rate at which the system fills and pressurizes to eliminate the possibility of creating any pressure transients from excessive liquid velocity. Flow may begin when the meter and system are filled, pressurized, and free of air.
To prevent damage to the meter or strainer due to foreign debris in the piping system, care must be taken when starting the flow. Foreign particles that were removed during flushing and hydrotesting will collect in the strainer basket. Monitor the strainer differential pressure for the first month of operation. If necessary, clean the strainer basket. Examine the strainer basket carefully and replace it is there is any damage. Ensure that the thermal relief valves are installed and functioning properly.