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Seeing double

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Oilfield Technology,


Steve Digby, SPX FLOW, UK, explains why double acting double diaphragm metering pumps are a perfect storm of benefits for offshore operators.

There are four things that are often critical for solutions in offshore applications: safety, reliability, footprint and weight. Although there are many different pump technologies available, SPX FLOW® is seeing an increasing preference for double acting double diaphragm (DADD) metering pumps because of the advantages they offer. Typically used in applications including polymer injection, methanol injection and hydrocarbon condensate metering, these pumps have proven reliability, accuracy and, with a double acting head, offer virtually double the capacity of a single acting pump in the same footprint. SPX FLOW has vast experience in developing and producing pumps and has a wide range of models in its portfolio. This article provides a brief overview of some of the advantages that DADD pumps have in the above referenced applications over alternative models.


Figure 1. Novalink display for 3 head machine.

Designed for safe operation

The hermetically sealed diaphragm in a hydraulic diaphragm pump head separates and protects the moving parts of the pump from the pumped liquid. It is pushed backwards and forwards by hydraulic fluid and, as it moves backwards, it increases the volume inside the pump chamber. This creates suction which opens an inlet valve and allows fluid into the chamber. As it is pushed forwards again the chamber volume is reduced, opening the outlet valve and pushing the fluid out again.

Once set to a given flow rate and pressure, the operation of a diaphragm pump is simple, and no further adjustments are required. As fewer parts are exposed to the pumped fluid, the overall reliability and availability of the pump increases. Indeed, the level of reliability may, dependent on the criticality of the process, negate the need for a duty/standby configuration. As well as being designed to avoid leaks with a hermetic seal, the lack of moving parts in a diaphragm pump configuration also means it provides quiet, energy efficient operation. Using a double diaphragm further increases protection against any leaks and produces a seal-less design that reduces required maintenance through the lifetime of the pump.


Figure 2. B+L Novados H4 pumps with DADD pumpheads.

Plunger pumps, with sophisticated sealing systems, are more prone to wear and tear because of the way they operate which can lead to the potential for leakage. Although a plunger pump may offer an initially lower Capex investment, the operating and longer-term maintenance of the pump means, for many applications, the overall return on investment is better with diaphragm technology.

Doubling capacity without the associated footprint

Space is always a premium on offshore assets and is often a challenge even on onshore oil and gas facilities. Wider use of FPSOs and the need for more technology to access remote reserves further means the footprint for pumping solutions is more critical than ever.

A diaphragm pump using a double acting head can effectively double the pumping capacity of a single acting pump. This is achieved as the displacer suction and discharge stroke operate simultaneously, giving the capacity of equivalent duplex pumps. Using just a single gear, the use of double acting pump heads further reduces maintenance time and cost compared with a duplex system. It also creates a more compact solution with reduced footprint and weight, typically in excess of 20% less than equivalent duplex pump heads, making it ideal for achieving required pumping duty where space is at a premium.

For installations that require increased capacity, there is the possibility to increase the pump head size, but this only provides limited expansion. A second head and gear can be added, but this requires more space. For existing single acting diaphragm pumps, however, the double acting head can be an easy retrofit without impacting the current footprint and requiring only minor adjustments to the pipework.

Accuracy and efficiency reduce costs and environmental impact

Of course, the inherent safety of the diaphragm design protects against hydrocarbons or chemicals escaping. This benefits both the surrounding environment and helps to keep operating personnel safe. The accuracy and efficiency of the diaphragm pump, however, further lowers costs and reduces impact on the environment in oilfield applications.


Figure 3. Duplex B+L Novados B pump with DADD pumpheads. 

Hydraulic diaphragm pump technology provides metering accuracies of up to ±0.5%. Combined with the ability to pump against high pipeline pressures, these pumps are ideal to ensure that the correct ratios are maintained in chemical injection applications. By adjusting pump speed or stroke length, the pumps are easily controlled between 0 and 100% flow while still maintaining accuracy. This gives flexibility to vary the volume of chemicals used to suit changing process needs. The high accuracy and flexibility provided optimises the use of expensive chemicals, saving operational cost, while ensuring quality is maintained with correct dosages used.

The efficiency of DADD pump technology also optimises power consumption. This results in the use of smaller motors, less ongoing power consumption and a more effective overall solution. Advanced digital control actuators, with a wide range of communication protocols to integrate with control systems, further improve the efficiency of pump operation and accuracy of response in line with process needs.

Reducing maintenance overheads and maximising availability

By using a DADD pump, operators gain the same capacity as double-acting plunger pumps or duplex diaphragm pumps. A plunger pump solution has benefits but does not offer the same protection against leaks as a double diaphragm pump. Using a duplex pump adds to size, weight and complexity. Both alternatives would also increase maintenance overheads compared with the DADD, which offers a combination of simplicity and, by reducing the potential failure points in the installation, reliability.

Other ways to reduce maintenance and increase pump availability for a DADD include using a continuous pump parameter monitoring system. The use of such software, which can be incorporated into an asset management system, enables remote, advance detection of possible pump failure, which can help optimise maintenance schedules, maximise uptime and protect vital production capacity.


Figure 4. Methanol Injection Unit containing duty/standby Novados B pumps with DADD pumpheads.

By using a continuous monitoring system to check pump parameters, any signs of wear can be identified and assessed. The software will also identify the precise fault location prior to any scheduled repair, which minimises downtime, lowers repair costs and maximises pump availability. The NOVALINK-CSM2 software used by SPX FLOW with its NOVADOS DADD pumps, measures the pressure in the hydraulic compartment of the pump using a pressure transmitter per pump head. It then creates real time pressure volume (pV) diagrams of the pump heads for each rotation of the pump crankshaft. These are used to calculate pump characteristics including efficiency level, average pressure, flow rate, etc. The software also uses simulation calculations to determine the ideal pump characteristics and pV diagrams, against which the measured pV diagrams can be compared. A traffic light system then gives operators an immediate indication of how the pump is performing and whether any maintenance actions are required.

Diaphragm pump applications

Because of the benefits a diaphragm pump offers to the oil and gas industry, there is increasing demand for diaphragm pumps for applications including methanol or LDHI injection, hydrocarbon condensate duty, glycol injection and enhanced oil recovery (EOR) polymer injection. Diaphragm pumps operate reliably under high pressure, high flow transfer conditions, and can handle a wide range of temperatures. Accessing declining reserves in mature fields requires technology that can increase recovery factors and handle higher pressures created from longer umbilical lines.

Recent examples include applications in the North Sea and some 160 pumps for fields in India. By using DADD technology, the operators benefited from 35 - 40% saving in space and weight for the pump package with reduced kW motor due to the efficiency of the pumps.

Summary

In offshore oil and gas applications, safety and reliability are of paramount importance, but space is a premium. Increasing pressures on costs and more marginal fields, further mean that cost-efficient operation is required. Hydraulic diaphragm technology offers a solution that combines all these challenges in a single, highly reliable and efficient metering pump solution.

Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/23042019/seeing-double/

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