New technological breakthroughs have enabled measurements of oil and water mixtures with some gas bubbles or infrequent gas pockets that have traditionally caused damage to other types of inline mechanically driven flow meters. The inline meter may be subject to sand, grit, stones (up to 0.25" diameter), solids and suspensions that could stop and damage other meters. This damage can be caused by close clearances, rotating seals, and stuffing boxes or sensor fouling , depending on the type of intrusive device implemented in the field. Continued or even brief exposure to these elements require removal of the flow meter from the line in question. In an effort minimize downtime, maintenance costs and overall cost of ownership, several oil enterprises have been implementing new technology dual mode clamp on ultrasonic flow meters in these very applications. High speed processors, advanced filtering software and intelligent sensor design have paved the way for a new generation of flow measuring devices currently implemented in oil fields around the world.

Although more durable designs and more cost-effective inline meters are being produced to work with dirty process fluid or where the risk of overspeeding is a potential problem, there are reasons that have persuaded certain groups of users to lean towards using newer digital based clamp on technology. The main criteria being the total cost of ownership. Although the initial costs of other inline devices are more attractive in the initial stages, there are long term and immediate expenses which have to be accounted for. Some of the problems associated with traditional methods are:

1. Continual re-calibration of flow devices due to mechanical wear and tear
2. Direct Damage to flow elements due to gas pockets , grit or other materials causing calibration deviations or complete failures within days, weeks or months.
3. Cost of installations , re-installations and manpower
4. Remote locations of these devices are a major source of frustration due to the distance involved and the need for continual maintenance and checking

Advantages of a clamp on design are evidenced by

1. Lower cost of ownership (no maintenance required, no moving parts)
2. Non-intrusive designs considerably speed up installation time
3. Re-calibration is not a continual requirement and does not require removal of elements from the line
4. Clamp on Flowmeters are immune to gas pockets , sand and grit since there is no contact with the process media
5. Reynolds compensation factors can be implemented in the software design to improve accuracy on liquids with fixed kinematic viscosities.

The following shows typical flow data gathered on a 10 inch crude oil line in the liquid phase with a fixed kinematic viscosity and density. It is important for the user to input viscosity and density parameters so that the change of state from laminar to turbulent flow can be predicted . The accuracies normally achieved are withing 1% of rate if pipe conditions are acceptable and correct process and pipe data are entered into the flow computer. Like many other types of meters, clamp on flowmeters require fully formed axially symmetric flow profiles, so reasonable lengths of straight pipe are required for more accurate flow measurements. Below is typical data gathered from the crude oil measurement over several hours

In this case, Transit Time methods are being used since Doppler methods cannot measure accurately at low flow rates due to limitations on the dependency of particles or gas bubbles in the line which may not even be present

Transit time meters work on the principal that the time of flight from the downstream transducer to the upstream during flow will always be greater than the upstream to downstream time. This is measured in milliseconds and can be correlated to flow velocities

Successful measurements require the user to input the pipe wall thickness, outer diameter, liquid kinematic viscosity and density if the fluid is an unkown type. This information can normally be obtained from an experienced laboratory or from previous analytical data.

DIFFICULT APPLICATIONS

The following is a typical case study on a difficult application where economics played a major role in technology consideration . We can consider this application as a “worst case” implementation where the meters were operating on their technological limits. Test locations where the flow was always in a liquid state have not presented problems for the technology with steady readings and accurate data measurements and clamp on flowmeters are accepted as an alternative means to accurate flow measurements.

EESIFLO does not claim to measure three phase flow regimes but the data gathered by the devices in these applications have proved useful to oil well planners. Some level of accuracy is achievable depending on pipe and liquid conditions. This application is of interest to users who cannot justify the purchase of full blown inline multiphase flowmeters that claim to accurately measure oil , liquid and gas ratios. In this instance, it was important for the client to obtain non-intrusive information from a well and from subsequent other wells. It was known that the flow maintained a liquid state for the most part but previous inline devices suffered continual damage and another solution was of most importance.

It was the intention of EESiFlo to work cooperatively with X oil company to solve the flow measurement problems experienced at various oil well locations which were predominantly oil based liquids containing mixtures of water for the most with intervals of high aeration and GVF at particular times of the day. Although it was impossible to measure the gas phases, the meters produced results that gave operators important information on the production of their wells which enabled them to plan for further courses of action

Following are observations from data gathered on a 4 inch carbon steel pipe flow applicaton

Above is a graphical representation of the Volumetric Flow (in barrels per hour) recorded during the initial installation at POV location. Note that a cyclic “noise” phenomenon occurred at approximately 3 to 4 hour intervals.

Below is a short section of data displaying numerous accurate flow values between 9.5 and 10.5 barrels per hour. Near the end of this period, note a period of erratic data which continued for ½ hour, until the data signal was eventually lost. This Lost Data period continued for approximately ½ hour when the signal returned. The reason for this (and subsequent) lost data is unknown, and is discussed later.

For the remainder of this data set, data appeared very accurate in three or four hour groups, at the end of which some undefined interference caused flow data to become erratic, and eventually the signal was lost. This loss of signal appears below on repetitive 3 to 4 hour cycles.

Loss of Signal

In addition to recording flow velocity (shown red below) the EESiFlo “Series” product line are capable of recording the signal strength of each data transmission. This signal strength is represented by a black line in the graph below. Note that prior to the loss both flow values and signal strength remain acceptable. As signal strength values dropped considerably, the flow values became erratic. Once signal strength returned to an acceptable level, flow values also became valid.

In addition to flow volume and signal strength, EESiFlo “Series” products are capable of measuring and recording the changing Speed of Sound of the medium, which is represented below by a blue line. Again, when flow data appeared normal, the signal strength and speed of sound all appeared normal. However, when the physical properties of the medium changed, all three signals became erratic until both speed of sound and flow values where finally lost. As soon as the properties returned to normal values, both speed of sound and flow rates returned to normal.