The components of a crude oil metering skid are chosen around four fluid properties: viscosity and how it varies with temperature, water cut, gas content and solids load. Meter technology follows from the viscosity range and line size; density and water cut instrumentation decide which data is taken online when gross volume is converted to net; and gas elimination, strainers, samplers and prover connections are added according to the gas, sediment and commercial requirements of the stream. This page looks at how skid hardware is selected for crude.
How measured quantity is calculated and verified for commercial purposes is the subject of the Crude Oil Custody Transfer Systems page. Packaged units that automatically transfer crude from a lease into a pipeline are covered separately on the LACT Units page.
Where crude oil metering skids are used
· Terminal and storage inlets and outlets: Metering crude received from trucks or pipelines and shipped out.
· In-plant transfer and allocation: Measuring production from several leases separately before it enters a common line.
· Refinery crude feed lines: Metering for process control and material balance; custody transfer may not apply.
· Truck loading and unloading points: Receipt and dispatch points where a meter ticket is issued per truck.
Meter technology and viscosity
Crude spans a very wide viscosity range, from near-condensate grades to heavy oils that barely flow at ambient temperature. The main meter types for liquid hydrocarbons are positive displacement, turbine, Coriolis and multipath ultrasonic, and each behaves differently on crude:
Assessing meter types for crude oil service
Meter type | Strength on crude | What to watch |
Positive displacement (PD) meter | Mechanically divides volume; widely used on viscous products | Moving parts are affected by sediment and sand; a strainer and maintenance plan are needed |
Turbine meter | High flow capacity on clean, stable, low-viscosity streams | Performance is viscosity-sensitive; meter factor may shift as product or temperature changes |
Coriolis mass flow meter | Measures mass, density and temperature together; no moving parts | Pressure drop rises with viscosity; size and cost can limit use on large lines |
Multipath ultrasonic meter | Low pressure drop on large, high-flow lines; provides diagnostics | Flow profile and viscosity effects must be assessed; straight run or flow conditioner required |
Viscosity is not the only criterion. Flow range, line size, allowable pressure drop, solids load, maintenance capability and how the meter will be proved in the field are weighed together. Where viscosity swings seasonally or crudes from different leases are commingled, how much meter performance shifts with that variation deserves its own review.
Density and water cut measurement
Converting gross volume to net standard volume requires density and S&W data. Density can come from an online densitometer, from the density output of a Coriolis meter, or from a laboratory value entered into the flow computer. Online density also allows mass to be inferred from observed volume.
A water cut analyzer tracks the percentage of water in the stream in real time. That is valuable for quality alarms, segregating high-water batches and operating decisions. For commercial calculation, however, the S&W value usually comes from lab analysis of a flow-proportional composite sample. Having both the analyzer and the sampler on the skid lets real-time monitoring and the commercial result complement each other.
Temperature, pressure and the flow computer
Crude volume changes noticeably with temperature. Temperature and pressure measured close to the meter are processed in the flow computer using generalized crude oil correction factors to bring volume to reference conditions. Recognized practice for electronic liquid measurement covers not only the calculation but also records, reporting, audit trail and data security. A flow computer that also handles meter factor linearization, sampler control and prover calculations makes the skid easier to run as one system.
Gas elimination, back pressure and strainers
To keep dissolved gas from breaking out inside the meter, pressure at the metering point must be held high enough. A back pressure or flow control valve downstream of the meter serves that purpose, and where flow is intermittent or fed from trucks an air eliminator is added upstream. The strainer protects meters, especially those with moving parts, from sediment and sand. The combined pressure drop of strainer, air eliminator and control valve is taken into account in pump selection.
Sampler and mixing
An automatic sampler integrated into the skid should sit at a well-mixed cross-section. Because water droplets can collect at the bottom of the pipe at low flow, a static or powered mixer is added where needed. The sampler runs flow-proportionally from a volume signal supplied by the flow computer. Sampling system design is covered on the Crude Oil Sampling Systems page.
Prover connections and spare runs
On commercial skids the meter factor must be determinable in the field, on the actual product and at operating conditions. Valved connections are therefore provided so that a fixed prover, a portable prover or a master meter can be connected. Being able to verify tight shutoff on the prover diverter valves assures that metered product does not bypass the prover. Where uninterrupted measurement is required, a second meter run takes over flow during meter maintenance or proving.
Standard scope and project options
Component | Function on crude | Standard / option |
Inlet and outlet isolation valves | Isolating the run for maintenance and emergencies | Standard |
Strainer | Protects the meter from sediment, sand and wax fragments | Standard; mesh chosen for the product |
Air eliminator | Prevents free gas from being metered | For truck-fed or intermittent flow |
Meter | Volume or mass measurement | Standard; technology by viscosity |
Temperature and pressure | Inputs for volume correction | Standard |
Densitometer | Online density and inferred mass | For custody or mass-based calculation |
Water cut analyzer | Real-time water monitoring and quality decisions | Project dependent |
Automatic sampler and mixer | Composite sample for S&W and quality | Required for custody transfer; optional elsewhere |
Prover connections or fixed prover | Field determination of meter factor | For custody transfer |
Back pressure / flow control valve | Single-phase flow and rate control | Standard |
Flow computer and panel | Correction, records, communication | Standard |
Heat tracing and insulation | Keeps viscous or waxy crude mobile | Depending on product and climate |
Design criteria
Parameter | Why it matters in design | Effect on selection |
Viscosity range and temperature dependence | Drives meter performance and pressure drop | Meter type, size, heating need |
S&W range | Net quantity and quality decisions rely on it | Sampler, mixer, water cut analyzer |
Dissolved gas and vapor pressure | Gas breakout in the meter causes error | Back pressure arrangement, air eliminator |
Flow range and tolerance to outages | Number of runs and redundancy | Single run, dual run or parallel runs |
Purpose of measurement | Process, allocation and custody transfer needs differ | Level of proving, sealing, records and redundancy |
H2S and sulfur content | Material compatibility and personnel safety | Wetted materials, gas detection |
Hazardous area and climate | Affects electrical and heating choices | Protection method, panel location, heat tracing |
TLY Enerji's scope
On crude metering skid projects, TLY Enerji provides engineering support for reviewing process data and selecting meters, density, temperature and pressure instruments and control valves suited to the product. Depending on scope, equipment supply, flow computer and control system integration, field installation, testing, commissioning and post-commissioning support can be provided. Which party covers items such as skid frame fabrication, the prover and the sampling system is clarified project by project.
Technical data for a skid proposal
· Product definition: density range, viscosity versus temperature, pour point
· Expected S&W range and solids content
· Minimum, normal and maximum flow; operating pressure and temperature
· Purpose of measurement: process, allocation or custody transfer
· Invoicing quantity: volume or mass
· Proving approach: fixed prover, portable prover or master meter
· H2S content, hazardous area classification and climate
· Control system interface and reporting requirements
Related pages
· Crude oil custody transfer systems: Commercial calculation of measured quantity, meter factor and audit.
· LACT units: Packaged units for automatic lease-to-pipeline transfer.
· Crude oil sampling systems: Skid-mounted sampler and mixing arrangements.
· Pipeline receipt and delivery metering: High-flow, multi-run continuous metering stations.
· Custody transfer truck loading skids: General layout of commercial metering skids for truck loading.
Crude metering skid FAQ
Can a turbine meter be used on crude with changing viscosity?
It can, but turbine meter performance is sensitive to viscosity, and the meter factor may drift as product or temperature changes. Such lines call for frequent proving across operating conditions, or for evaluating a technology less affected by viscosity. The decision is made together with the viscosity range and the proving options available.
Does a Coriolis meter remove the need for a densitometer?
Because a Coriolis meter measures mass, density and temperature at the same time, a separate densitometer is often unnecessary. Whether its density output can be used in commercial calculations, however, depends on the contract and on how density measurement will be verified. Some projects still require an independent densitometer or laboratory check.
Does a process-only crude skid need prover connections?
Not necessarily. For refinery feed or in-plant monitoring the meter can be removed for periodic calibration or checked in place against a master meter. If commercial use is possible in the future, though, adding prover connections at the outset is usually easier than modifying the skid later.
How is a skid protected on waxy crude?
Product temperature is kept safely above the pour point so wax does not deposit in the meter, strainer and instrument connections. Heat tracing and insulation, drain or flush provisions for long shutdowns and minimized dead volumes are evaluated. Heating temperature is set within the limits of both the product and the meter electronics.