Cryogenic loading racks are designed to move cryogenic liquids from storage to a transport vehicle efficiently, accurately and safely. Even the finest loading equipment, however, will not overcome flaws in the transfer system design and operation. Problems such as low pressure and low flow rate can affect loading rack performance and ultimately can compromise transfer accuracy and safety. Avoiding these problems requires the use of modern metering and control devices installed in the proper configurations.
Cryogenic liquids typically are composed of gases such as nitrogen, oxygen, argon, hydrogen and helium, as well as specialized products, such as nitrous oxide, liquefied natural gas and carbon dioxide. Unless these liquids are properly cooled and pressurized, they will revert to a gas rapidly through a process called flashing. In a best-case scenario, flashing will cause measurement errors that can result in an inaccurate loading and lost profits. In severe cases, flashing can cause thousands of dollars in equipment damage.
To prevent flashing and ensure an accurate loading, the liquid must fill the pipe completely during transfer. One way to make sure that the pipe remains filled is by using a turbine flowmeter, which is a metering device that incorporates a bladed turbine rotor installed in a flow tube (figure 1). The rotor is suspended axially in the direction of flow through the tube and acts as a transducer to sense the momentum of the flowing stream. The bladed rotor rotates on its axis in proportion to the rate of cryogenic liquid flowing through the tube. If the liquid pressure increases above or drops below a certain predetermined point - which depends on the liquid being transferred - the flowmeter will adjust the other parameters automatically to compensate for the change (table 1). Most turbine flowmeters have a linearity of ±0.5 percent or better, and a repeatability of ±0.1 percent at any point on the linear range.
Because turbine flowmeters are velocity-sensitive devices, a distance of 3' has been designed into the system on either side of the flowmeter to slow the velocity on the upstream side, where it otherwise would approach the flowmeter too quickly, and to speed up the velocity on the downstream side, where it otherwise would move too slowly. This configuration ensures a continuous, consistent flow through the flowmeter. By following the recommended design for this application, the process engineer can optimize the performance of the flowmeter and ensure accurate, efficient loading of the cryogenic liquid.
The Problem with GravityWith most loading racks, the liquid is pumped from storage to the vehicle. In some cases though, the product is moved through a gravity feed - without using a pump. This type of transfer can create some problems for cryogenic products and turbine flowmeters.
In a gravity-feed system, the pressure to move the product and keep it in liquid form is produced by the level of liquid in the storage tank. As this level drops, the pressure on the product is reduced, increasing the risk of flashing.
A lower pressure also reduces the flow rate. If the flow rate falls below the recommended minimum flow of the turbine flowmeter, the rotor within the flowmeter will fail to turn, and the device will not be able to obtain an accurate reading.
These problems can be overcome by ensuring that the supply tanks are located above the loading positions. A backpressure device such as a discharge valve shown also should be installed downstream of the turbine flowmeter to ensure that the flowmeter stays full. If the backpressure device is set properly, it will not allow the fluid flow to fall below the minimum required by the turbine flowmeter, and it will keep pressure on the product to prevent flashing.
An Accurate MeasurementTrucks that are loaded with cryogenic loading racks that do not have flowmeters installed must drive across a scale to verify loading through weight. Purchasing and maintaining a scale onsite can be costly. If a scale is not on the premises, the trucks must drive to another location to verify weight, which wastes transit time. Scales also have had problems with accuracy due to wind, rain and ice. Overweight vehicles can be stiffly fined, and time simply cannot be made up.
When properly installed, turbine flowmeters can be accurate to within 50 lb on a load of 40,000 lb. While the flowmeters must be calibrated annually, such calibrations are less expensive than trying to verify loading through vehicle weight, potentially losing profits or paying fines due to inaccurate loadings.
By using correctly installed metering technology, plants handling cryogenic liquids can minimize errors, maximize loads and deliver product efficiently.