Meter tube inspection on natural gas pipeline

Background

Meter tubes are the “cash register” of the gas transmission industry.  A meter is installed at every point-of-custody transfer of the gas, both between gas companies and from the final gas company to the end user, such as a utility. There are several types of meters; one of the most common is an orifice meter. The orifice meter calculates gas flow across the orifice by measuring the pressure drop, or pressure delta.

An orifice meter consists of a straight run of pipeline with an orifice plate installed vertically in the center of the tube.  The diameter of the tubes varies from 2 in. to 24 in. or larger, and the diameter of the orifice in the plate varies accordingly. Upstream of the orifice plate there is a set of smaller diameter tubes installed inside the pipeline which resemble a short, straight heat exchanger bundle. These are straightening vanes, and their purpose is to smooth out the flow of gas before it passes through the orifice. Smooth flow is essential for accurate measurement.

How important is accuracy? (And why would anyone spend $15K – to $30K on remote visual inspection equipment to determine it? Consider a utility boiler that operates on natural gas. If the boiler uses 1,000,000 SCFH*, (at $3.50 per 1000 SCF** @100 PSI), having just a 1% measurement error, the owner can stand to lose nearly $850.00 per day (or $25,000 per month, or over $300K per year!) Spending money on RVI to maintain meter measurement accuracy can be justified by the magnitude of the bill, and potentially large negative consequences that could occur if the measurement is in error.

*SCFH = standard cubic feet per hour, **SCF = standard cubic foot

Inspection

Meter tubes have high-pressure gas flowing through them. The gas company contact you are working with (typically a measurement technician or measurement supervisor) will take the following steps (or similar steps) before proceeding with any inspections:

1.    Isolate the meter by shutting off valves at both ends, and/or diverting the gas flow to a parallel flow meter station.

2.    “Blow down” (relieve the gas pressure in the meter by opening a blow down valve).

3.    Check the area for safe operating conditions, gas pockets, etc. 

The primary objective of the inspection is to determine that the straightening vanes are clear.  Fouled vanes can introduce turbulence into the flow, potentially reducing measurement accuracy. With a fiberscope or XL Go+, the probe is inserted into a blow down port or tap, or on some meters the orifice plate can be removed, and the scope can be inserted into the orifice plate area. Ideally, a view from the downstream side is desired because that is where the most buildup or fouling occurs.

Depending on the diameter of the meter tube, articulation can be used to see through each of the individual straightening vane tubes. If the meter tube is too large, use a guide tube to help with positioning. As the meter tube diameter increases, the available blow down or taps will be further away from the straightening vanes. This is a result of the meter tube construction specifications relating to structural strength requirements, and the concept of any holes or welds weakening the structure. If using a rigid borescope, select the largest diameter possible (side viewing), and the tightest field of view possible – you will be looking a long way.  Some very large diameter meter tubes may require backlighting with a second light source. 

Scope view of straightening vanes. Observe the fouling on vane at the 9 o’clock position.	Measurement errors can result if debris is trapped upstream of straightening vanes.