Positive displacement motor design

Figure 4-201. Basic positive displacement motor design principle. (Courtesy Smith International, Inc.)...

There are, of course, variations on the downhole positive displacement motor design, but the basic sections discussed above will be common to all designs. 

High torques and low speeds are obtainable with certain positive displacement motor designs, particularly, the higher lobe profiles (see Figure 4-203). 

If the above performance parameters for a positive displacement motor design are known for a given circulation flowrate (denoted as 1), the performance parameters for the new circulation flowrate (denoted as 2) can be found by the following relationships ... 

In this section the design and the operational characteristics and procedures of the most frequently used downhole motors will be discussed. These are the downhole turbine motor and the downhole positive displacement motor. 

The dump valve is a very important feature of the positive displacement motor. The positive displacement motor does not permit fluid to flow through the motor unless the motor is rotating. Therefore, a dump valve at the top of the motor allows drilling fluid to be circulated to the annulus even if the motor is not rotating. Most dump valve designs allow the fluid to circulate to the annulus when the pressure is below a certain threshold, say below 50 psi or so. Only when the surface pump is operated does the valve close to force all fluid through the motor. 

In general, the larger lobe profile number ratios of a positive displacement motor, the higher the torque output and the lower the speed (assuming all other design limitations remain the same). 

The rotor of the Moineau-type positive displacement motor has a helical design. The axial wave number of the rotor is one less than the axial wave number for the stator for a given chamber. This allows the formation of a series of fluid cavities as the rotor rotates. The number of stator wave lengths n and the number of rotor wave lengths n per chamber are related by [79,86]... 

It should be noted that the positive displacement motor performance parameters are independent of the drilling mud weight. Thus, these performance parameters will vary with motor design values and the circulation flowrate. 

The positive displacement motor whose performance characteristics are given in Table 4-115 is a 5 6 lobe profile motor. This lobe profile design is usually used for straight hole drilling with roller rock bits, or for deviation control operations where high torque polycrystalline diamond compact bit or diamond bits are used for deviation control operations. 

A 61-in. outside diameter positive displacement motor of a 1 2 lobe profile design (where performance data are given in Table 4-114) has rotor eccentricity of 0.60 in., a reference diameter (rotor shaft diameter) of 2.48 in. and a rotor pitch of 38.0 in. If the pressure drop across the motor is determined to be 500 psi at a circulation flowrate of 350 gal/min with 12.0 Ib/gal, find the torque, rotational speed and the horsepower of the motor. 

The positive displacement motor of the Moineau-type design can be operated with unstable foam (or mist) as the drilling fluid. Some liquid must be placed in the air or gas flow to lubricate the elastomer stator as the metal rotor rotates against the elastomer. Positive displacement motors have been operated quite... 

It is cautioned that NEMA Design B motors (normal starting torque) may not be suitable for applications requiring high starting torque such as positive displacement pumps. NEMA Design C motors should be used in this service. 

To make up the difference between the expected expander returned power and the required compressor input power. Meruit designed an air system using a commercially available positive-displacement compressor driven by an electric motor in tandem with Meruit s turbocompressor. Meruit designed a family of compressor and expander wheels with var5dng characteristics (specific speed, dimensional ratios, etc.) representing different compromises in providing performance compatible with the DOE specifications. After extensive evaluation, a compressor wheel was chosen to provide a desired pressure curve over a wide range of mass flows, and a turbine wheel was matched to it. The chosen compressor wheel is intended to trade peak efficiency for extended turndown performance. 

The solvent delivery system in this instrument uses a syringe-style, positive-displacement, continuous delivery design. Each syringe is motor driven and software controlled so that crossover-related flow phenomena are eliminated. Gradients can also be performed, and are mixed under high pressure to minimize the gradient delay. 

Two positive displacement WUson-Snyder pumps were installed at the main pump station. These were Mven by 746 kW (1000 hp) direct current motors. The booster station is connected to a water pond and draws water on demand to avoid slack flow by providing additional back pressure in low flow conditions. Choke stations are also provided for additional back pressure. According to Weston and Worthen, the pipeline used high-yield-strength steel rated at 413 MPa (60,000 psi). An allowance of 2.5 mm (0.1 in) for corrosion/erosion over a lifetime of 25 years was factored into the design. The thickness of the pipeline varied between 6.4-12.7 mm (0.25-0.5 in). 

For the purposes of sizing the motor and determining the screw lead during the design process, a sinusoidal desired position was assumed with the maximum peak-to-peak displacement of 5 cm and a frequency, /r, of 0.5 Hz. In this case, the frequency of the sinusoidal desired position is denoted as ... 

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