NGL Meter Station Design

1. Overview

NGL meter station design is not a one-size-fits-all scenario. Multiple considerations influence the station design, and all must be taken into account. The major considerations are:

• What product or products will be measured
• What meter technology to utilize
• Process design limitations

Product Classification

2. Measurement Types

Volumetric Measurement

Volumetric measurement is used for purity products where composition is known and consistent. Volume products are sold at net conditions but measured at flowing conditions.

Net conditions are defined as equilibrium vapor pressure (EVP) at 60°F, or at 14.7 psia if the EVP of the product is less than 1 atmosphere.

To correct the product to net conditions, accurate measurement of temperature and pressure is required. These process measurements are used to calculate the CTL (Correction for Temperature of Liquid) and CPL (Correction for Pressure of Liquid) from the API MPMS product tables.

Inferred Mass Measurement

Inferred mass measurement uses a volumetric meter coupled with an online densitometer. The indicated volume is multiplied by the live flowing density to calculate the mass flow rate:

Direct Mass Measurement

Direct mass measurement uses a Coriolis meter configured to output mass directly. The Coriolis meter measures mass flow rate based on the inertial force exerted on a vibrating tube as fluid flows through it.

The meter can also measure the period frequency of the fluid to calculate density. For direct mass output, the Coriolis meter functions as both the meter and the densitometer.

3. Meter Technologies

Turbine Meters

Turbine meters are mechanical meters that convert fluid velocity into rotational velocity. Any rotational velocity component in the product stream (swirl) can affect the meter, so flow conditioning should be installed as a good design practice.

Positive Displacement (PD) Meters

PD meters are mechanical meters that rely on small, discrete volume pockets to measure the volumetric flow rate. They are highly repeatable and work well in flow regions and viscosities that could be problematic with other technologies.

Coriolis Meters

Coriolis meters are non-mechanical and rely on the inertial force exerted on an object as a result of movement relative to a rotating frame of reference. As flow passes through a vibrating tube, the tube begins to torque proportionally to the mass flow rate.

Meter Technology Comparison

4. Flow Conditioning

Meter performance can be negatively affected by flow profile distortions (swirl) created by piping effects prior to entering the meter. A general rule of thumb is that 30 pipe diameters of straight pipe is required to straighten most flow profile distortions.

Flow conditioners can eliminate the flow distortion in as little as 10 pipe diameters. Types of flow conditioners range from simple tube bundles to high performance flow conditioners which address both swirl and velocity profiles.

Straight Pipe Requirements

D = pipe diameter

5. Back Pressure Control

A back pressure control valve should be installed on each meter run to guarantee the pressure is maintained above the minimum required to prevent vaporization.

6. Density Measurement

The product's temperature is required for CTL and CPL determinations. This is accomplished by two philosophies:

1. Fixed specific gravity at 60°F - Does not require a densitometer
2. Live flowing density - Requires a densitometer. From the flowing density along with temperature and pressure, the flow computer uses an iterative procedure to back calculate the SG at 60°F

Densitometer Types

Insertion Densitometers

Insertion densitometers have velocity limitations which are commonly exceeded by product velocities. In this case, the densitometers are installed in stilling wells off the mainline pipe. Stilling wells limit product velocities but raise concerns about maintaining a representative sample.

Flow-Through Densitometers

Flow-through densitometers (vibrating tube type or Coriolis meter density function) are installed in a sample loop. Scoops or quills should be utilized to ensure the sample is taken from the center third of the meter run piping.

Facilities (pycnometer connections) must be installed to verify the density measurement. During density verification, temperatures at the densitometer outlet, mainline pipe, and pycnometer outlet must be within 0.2°F.

7. Meter Proving

For custody measurement, provisions must be installed to prove each meter run independently. The decision should be made whether the meter will be proved with a portable prover or with a permanently installed prover.

Prover Selection Factors

• Frequency of provings
• Number of meters to be proved
• Location of the site
• Operating cost vs. capital cost

Prover Operation

A prover is a device calibrated to a known volume between a set of switches. Two commonly utilized types are piston provers and ball provers. These devices calibrate the meter by accumulating the pulses generated by the meter for the known volume of the prover.

Proving Considerations

• Piping to the prover should be minimized to keep operating conditions close to the meter
• Caution when volumetrically proving Ethane and Y-grade - CTL and CPL calculations can be in error for certain process conditions
• Turbine meters require re-proving when product changes
• Coriolis meters have phase shift between measurement and prover that can affect repeatability

8. Composite Sampling

The primary difference on a mass system is the installation of a flow-weighted composite sampler. A composite sampler is required to ensure the components within the stream are properly accounted.

Sampler Installation

• Located as close to the piping as possible
• Minimize sample volume in quill, pump, and tubing
• Use small tubing to minimize sample lag
• Smaller, more frequent bite sizes are better than larger, less frequent

Accumulator Sizing

Accumulator pots should be adequately large enough to accommodate the sample being collected. Sampler pacing should be set such that the composite sample collected is representative of what flowed through the meter.

Dual sampler systems may be required to facilitate sample collection and analysis per contracts. Once analyzed, the analysis is applied to the totalized mass batch for component accounting.