Prognosis - A Unique, Powerful, Industrially Proven Condition Based Monitoring (CBM) System for DP Meters

Prognosis is the worlds first and only Condition Based Monitoring system for Differential Pressure Meters. Using patented techniques, Prognosis is capable of real-time diagnostics and is extremely powerful in assuring good flow measurement and detecting issues with any standard DP Meter.

Brief Theory Introduction

The essential components to any DP meter are the primary element placing a constriction in the pipe, causing a drop in fluid pressure and a DP Transmitter measuring that drop in pressure which is used to predict the rate of flow through the meter as shown in Figure 1 below.

prognosis orifice

By utilising a third taping downstream of a meter body (P3), it is possible to read three differential pressure measurements (Figure 2 below).

These three DP measurements are the core of the Prognosis system, unlocking the full self-diagnostic capabilities of the meter.

prognosis orifice triple tap

The three DPs are:

  • the traditional meter DP (DPt): P1-P2
  • the recovered DP (DPr): P3-P2
  • the permanent pressure loss; DP (DPppl): P1-P3

Using the three DP readings, the pressure field through the meter is monitored and the three DPs are compared in multiple ways using other process conditions and meter geometry, providing powerful information on the meter’s performance using purely information which is already an integral part of the meter without the need for any inspection.

Simplified graph of pressure field through a DP meter

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The diagnostic calculations are derived from well understood physical principles and industry standard calculations for DP meter flow rate. Seven diagnostic results are calculated and monitored in real time for each meter. The results are displayed in the Prognosis software as 4 points plotted on an x,y-axis (x1,y1), (x2,y2), (x3,y3) and (x4,0) and a 1x1 unitless box. Due to normalisation (each comparison result is divided by its comparable uncertainty), if the meter is operating correctly to within its allowable uncertainties, each of the seven diagnostic results will be between [-1, +1]. In other words, each of the 4 points will be inside the box.

Figure 4: Prognosis User interface showing no meter issue (all points inside the box)

If this is not the case then the Prognosis system raises an alarm and the Operator is made aware that there is an issue with the meter system.

Figure 5: Prognosis User interface showing a meter issue detected; in this case an issue with the meter DP

Differing Meter Types

Prognosis can be applied to ANY DP meter measuring single phase liquid, single phase gas or wet gas flows as long as a suitable downstream straight wall Pressure tap is available (see Section 4 below). Examples of DP meters are: Orifice, Venturi, Cone, Flow Nozzles, Wedge meters.

Figure 6: Diagram showing pipe section with Venturi primary element, 3 pressure tappings and 3 DPs

Figure 7: Diagram showing pipe section with Cone primary element, 3 pressure tappings and 3 DPs

Wet Gas Flow

Wet gas induces a positive gas flow error on a meter and creates a specific pattern on the diagnostic plot. The diagnostic plot can be zeroed to a specific wet gas flow, thereby indicating when the liquid loading changes (either positively or negatively) and hence when a new spot check is required.
Wet gas flow adversely affects the DP readings – hydrates, scale, wax and salts can block taps, transmitters can be easily saturated etc. However, as for single phase applications, Prognosis will alert the operator when there is a problem with one or more of the DP measurements, regardless of liquid loading or any other issue with the meter.
Wet gas flow conditions can change periodically. The diagnostics offer a user friendly immediate alarm and visual indication of a changing flow condition, e.g. changes in the liquid loading.

The Downstream Pressure Tap (P3)

The downstream Pressure Tap (P3) is ideally a standard straight wall pressure tapping, with no protrusion in the pipe and which is dedicated for the purpose of the Prognosis system.
Ideal systems will have no protrusions in the pipe between the existing P2 and the downstream Pressure tapping, P3.
For Orifice meters the downstream Pressure Tap is ideally 6D (6 x internal Pipe diameter) downstream of an Orifice plate.
It is possible to compensate for tapping points which are more than 6D downstream of the meter, however, various factors need to be considered including:
• Are there any protrusions in the pipe or any take-offs upstream of P3 (and downstream of P2)?
• How much further than 6D downstream is the proposed tapping point?
For Venturi and Cone meters (meters which need to be calibrated prior to installation, regardless of whether or not Prognosis is implemented), the downstream Pressure tap can be closer than 6D to the meter‘s convergent section. It is however important that the meter is calibrated with the downstream spool piece and instrumentation arranged as it will be in the field.
It MAY still be possible to apply Prognosis to a Venturi/Cone meter which is already in the field and whose calibration did not include downstream Pressure readings.

Typical Faults Identified

Below is a list of typical system faults which will trigger an alarm within the Prognosis system:

  • Incorrect Inlet or Throat Diameter In Use
  • Two-Phase Flow
  • Excessive Flow Disturbance Upstream of the Meter
  • Contamination Build-Up Through the Meter
  • Blocked Impulse Lines
  • Saturated DP Transmitter
  • Drifting DP Transmitter
  • A Buckled Orifice Plate
  • An Orifice Plate Installed Backwards
  • A Worn or Damage Orifice Leading Edge
  • A Damaged Cone Element
  • Incorrectly Spanned DP Transmitter, etc.
  • Incorrect Discharge Coefficient In use (Venturi/Cone meters)
  • Debris Trapped at Meter Throat
  • Meter Incorrectly Installed

Differing Data Sources/Architectures

The Prognosis software has the capability of interfacing to various third party systems or equipment via different communication protocols and this is configured for each meter stream prior to delivery.
Prognosis provides very flexible options for data acquisition from, for example, flow computers, I/O modules, DCS / SCADA systems or via an ERP (Enterprise Resource Planning) system.
Current Supported protocols are Modbus (Master or Slave) or OPC Server, utilising either Ethernet or Serial connections. Bespoke solutions can be implemented e.g. – Use of CSV files.
Below are two example architectures showing data acquisition from flow computers and data hand off to a higher level control system.

Figure 5: Example architecture with Prognosis acquiring data directly from Flow Computers

Figure 6: Example architecture with Prognosis acquiring data from Flow Computers via a high precision I/O device

Differing Reporting Methods

In order to report the findings of the Prognosis system as efficiently as possible to the Operator, Prognosis can:

  • Hand off Alarm Statuses and results to a Flow Computer, Supervisory System or Higher level Control System (serial/ethernet Modbus and OPC)
  • Generate Reports periodically on the status of the meter(s) being monitored by the Prognosis system (and store the reports for auditing purposes)
  • Send emails automatically when an alarm is raised
  • Send SMS message automatically when an alarm is raised
  • Email Reports periodically, providing a Summary of each meter’s diagnostic results during that period

Benefits

Prognosis provides the Operator with assurance of meter accuracy and auditable evidence meaning that the Prognosis system can form the basis of a Condition Based Maintenance strategy as recommended by DECC in the latest Hydrocarbon Measurement Guidelines for Operators in the UK:

prognosis differential pressure diagnostics

The benefits of being alerted in real time of potential measurement issues are clear. Below is an example financial benefits calculation:

Example Calculation

An orifice meter has an average flow rate over 1 year of 1,000,000 Sm3/d
Assumed gas price is £0.21/Sm3. Therefore the value of product is approximately £210,000/day, or approximately £75m/year.
Let’s say there is a problem with the meter system which is causing it to mis-measure, but the problem is small enough (1% of flow rate) that it goes undetected for 3 months until the next routine maintenance checks.
Cost of mis-measurement = 1% of cost of product for 3 months
= 1% of £18,750,000 = £187,500 (or £2,083 per day)
However, with a Prognosis system in place to monitor the meter’s performance, this 1% mis- measurement is detected straight away and the Operator is able to act to correct the problem, therefore saving £187,500
The cost of installing Prognosis on this meter could be in the region of £50,000 (including license + engineering of £10,000 and allowing £40,000 for the purchase and site installation of 2 DP Transmitters).
Therefore, in this particular example:
Savings in Year 1 = £137,500 Payback = 24 days

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If you require further information on this product or would like a quotation, please contact dp-flow on:

email: 
sales +44 1923 462500

Supplied by DP-Flow