Guideline Centrifugal Pump Selection and Piping Design For ...

The purpose of this article is to present guidelines and simplified techniques for sizing pumps and piping used in mud systems. In cases of unusual circumstances, such as lengthy or complex pipe runs, or when very heavy or viscous drilling muds are utilized, a qualified engineer should conduct a detailed analysis of the system and calculate an exact solution.

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Definitions

Total Head

When discussing pumps, it's important to use terminology that is well understood. For instance, the label on the left side of any centrifugal pump curve represents Total Head Feet. What does this imply?

The simplest way to allow water to flow is to lay a pipe on level ground connected to a standpipe (Figure 1). Water from the standpipe will flow steadily through the pipe, with a faster flow rate corresponding to a greater depth in the standpipe. The depth of water measured from the standpipe to the pipe connection centerline is referred to as Total Head.

Total Head remains constant for a specific pump operating at a constant speed, regardless of the fluid being pumped. However, as fluid density (mud weight) increases, the pressure exerted by the pump will also increase, as illustrated by the following relationship:

PMUD(PSIG) = PWATER(PSIG) x {FLUID DENSITY(LBS/GAL)}/8.34

If a pump generates 200 ft-hd, it follows that the pressure when pumping water will be:

PWATER = 200 FT-HD x 0.433 (PSIG/FT) x {FLUID DENSITY(LBS/GAL)}/8.34

PWATER = 200 FT-HD x 0.433 x 8.34/8.34 = 86.6 PSIG

NOTE: Fresh water weighs 8.34 lbs/gal.

When pumping 16 lb mud, the pressure will be:

PMUD = 200 FT-HD x 0.433(PSIG/FT) x (16/8.34)LBS/GAL = 166.1 PSIG

This indicates that the pump pressure nearly doubles. Consequently, the required pump horsepower increases proportionally. If the pump required 50 HP for water service, the horsepower requirement for 16 lb/gal mud is calculated as follows:

HP2 = HPWATER x (16/8.34) LBS/GAL = 50 x 1.92 = 95.9 HP

In summary, a pump's Total Head remains constant for any fluid pumped; only the pump pressure and horsepower requirements will vary. Therefore, the pump motor should be sized according to the heaviest weight mud necessary for pumping.

Pressure Head

Pressure Head represents the distance in feet that water will rise in a sight tube connected to a pipe containing liquid (see Figure 1).

In our example, the required desilter pressure head is 75 ft for any mud weight. However, the pressure would be 30.3 PSIG for water, 43.6 PSIG for 12 lb mud, and 58.1 PSIG for 16 lb mud. A useful rule of thumb states that the required pressure (PSIG) equals four times the mud weight (12 LB/GAL x 4 = 48 PSIG).

Fluid

The term fluid generally refers to water, brine, mud, oil, or any other liquid being pumped.

Flow Rate

Flow Rate is expressed in gallons per minute, barrels per minute, or cubic feet per second.

Velocity (V)

The velocity of the fluid in the pipe is the average velocity across the inside diameter and is expressed in FT/SEC.

Velocity Head

Velocity Head represents the energy required to accelerate the fluid from 0 FT/SEC in the suction tank to the velocity of the fluid in the pipe.

VELOCITY HEAD = {(V)2}/(2G), where G = (GRAVITATIONAL CONSTANT) = 32.2 FT/SEC/SEC.

The following charts can be utilized for velocity head ranges of our design fluid velocities.

Velocity (V)
Velocity Head (VH)
5 FT/SEC
0.39 FT-HD
10 FT/SEC
1.55 FT-HD
15 FT/SEC
3.49 FT-HD
20 FT/SEC
6.21 FT-HD

Velocity head is generally not a large number but should be included in pump calculations.

Example Problem

We will size a pump and piping for desilter service. The desilter is a typical 16 cone unit equipped with 4 inch diameter hydrocyclones.

STEP ONE

Determine the required pressure head and flow rate. If the pump is to supply a device such as a mud mixing hopper or desilter, consult the manufacturer's information or sales representative to determine the optimum flow rate and pressure head required at the device. For devices like desilters, the pressure head losses downstream are usually negligible and disregarded.

Our desilter cones require 50 GPM each at an inlet pressure head of 75 FT-HD. Therefore, the total capacity needed is:

16 CONES x 50 GPM = 800 GPM AT 75 FT-HD

STEP TWO

Select the basic pump to achieve the desired flow rate. It's advisable to refer to a manufacturer’s pump curve for the specific pump (See example - Figure 3).

If a curve is unavailable, the chart below provides reasonably accurate values for typical centrifugal pumps used for mud service.

The values in Table 1 represent the approximate maximum capacity and pressure head (FT-HD) for a given pump and speed.

Table 1: Capacity and Pressure for Various Pump Sizes

Pump Size
Maximum Nominal Capacity Pressure in Mud Service
2*3 RPM
200 GPM at 80 FT-HD
2*3 RPM
400 GPM at 175 FT-HD
3*4 RPM
450 GPM at 80 FT-HD
3*4 RPM
750 GPM at 175 FT-HD
4*5 RPM
800 GPM at 70 FT-HD
4*5 RPM
GPM at 160 FT-HD
5*6 RPM
GPM at 60 FT-HD
5*6 RPM
GPM at 160 FT-HD
6*8 RPM
GPM at 80 FT-HD
6*8 RPM
GPM at 175 FT-HD

The pump’s impeller may be machined to a smaller diameter to reduce pressure for a specific application. Consult the manufacturer's pump curves or representative to determine the optimal impeller diameter. It's important to avoid excessive pressure and flow for several reasons:

• The pump wears down at a faster rate.
• Equipment such as hydrocyclones will wear out more quickly.
• Hydrocyclones may not operate effectively at pressures higher or lower than specified.
• A larger electric motor, cable, and starter may be unnecessarily required.
• Operating the pump will incur greater energy costs.

The pump must generate more than 75 FT-HD for the desilter inlet, and its capacity must be at least 800 GPM. Therefore, one of the following pumps should be considered from the list: 4 x 5 Pump RPM and GPM at 160 FT-HD; or 5 x 6 Pump RPM and GPM at 160 FT-HD.

STEP THREE

Size the piping

The pump suction and discharge piping typically have the same diameter as the pump flange diameters. This ensures that fluid velocities are within the recommended ranges of 4 to 10 FT/SEC for suction lines and 4 to 12 FT/SEC for discharge lines. Specific circumstances may necessitate different pipe diameters, but it's advised to adhere to these velocity guidelines. Using smaller discharge piping can lead to increased pressure drops, affecting the pump's ability to deliver the required amount of fluid. (For instance, avoid using a 4 inch discharge pipe on a 6 x 8 pump and expecting full fluid flow.)

Refer to the attached tables (Figure 4) to find the proposed pipe diameter and flow rate, and verify that the velocities are reasonable.

For our desilter flow of 800 GPM, note from the attached tables:

For 4 inch pipe:
Velocity = 20.2 FT/SEC
Velocity Head = 6.32 FT
Friction Loss per 100 FT Pipe = 32.4 FT

For 5 inch pipe:
Velocity = 12.8 FT/SEC
Velocity Head = 2.56 FT
Friction Loss per 100 FT Pipe = 10.22 FT

For 6 inch pipe:
Velocity = 8.88 FT/SEC
Velocity Head = 1.23 FT
Friction Loss per 100 FT Pipe = 4.03 FT

For 8 inch pipe:
Velocity = 5.13 FT/SEC
Velocity Head = 0.41 FT
Friction Loss per 100 FT Pipe = 1.02

From this data, we can conclude:

• Use of 4 inch pipe is not recommended for either suction or discharge due to excessive velocity and pressure drop.
• 6 inch piping may be suitable for suction since it is relatively short and straight, with the pump's suction always flooded. It is also a good choice for discharge as the intended header is likely 6 inch pipe.
• 8 inch piping could be used for suction (V = 5.13 FT/SEC) as it still exceeds the minimum of 4 FT/SEC. This would be preferred if suction lines are long or if the suction pit fluid level is low compared to the pump.

We will choose 6 inch pipe for both the suction and discharge piping and will proceed with the 5 x 6 Pump - RPM.

STEP FOUR

Next, we will determine the total pressure head requirement of our piping system:

Total Pressure Head = Velocity Head + Pipe Losses + Vertical Head + Head Requirement of Desilter

Velocity Head

VEL-HD = 1.23 FT for 6 inch pipe with velocity = 8.88 FT/SEC (from Step Three).

Vertical Head

According to Figure 2, the vertical distance from the pump to the desilter inlet is 7 FT.

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Thus, the vertical head requirement = 7 FT.

Pipe Losses

We will assume that we have 60 FT of pipe, four elbows, and one butterfly valve in the system. You may utilize Figure 3 for a simplified solution.

FIGURE 3. Friction loss for water or mud in feet head per 100 feet of pipe (f = 0.03) with or without pipe fittings.

For 6 inch pipe, locate the velocity that is closest to 8.88 FT/SEC on the chart. It appears that 8.88 is listed. The friction loss per 100 FT of pipe with fittings is 11.01 FT. Since we only have 60 ft of pipe, our friction loss is:

11.01 FT x (60/100) = 6.6 FT-HD

For a more precise calculation, the loss from pipe fittings can be obtained from Figure 4.

FIGURE 4. Friction loss for water or mud in feet head per 100 feet of pipe (f = 0.03) with or without pipe fittings.

Main Features of f mud pump Products

The oil drilling mud pump, also referred to as the oilfield mud pump, is an essential component of drilling equipment, utilized to transport mud, water, and other flushing media within the drilling operations.

Characteristics of Oil Drilling Mud Pump Products:

• Capable of transporting high concentrations, high viscosity PaS, and suspensions containing particles.

• The liquid flow is stable without overcurrent pulsation or agitating and cutting phenomena.

• Discharge pressure is independent of rotational speed and can maintain high discharge pressure even at low flow rates.

• The flow rate is proportional to the rotational speed, with flow control achievable through a variable speed mechanism or speed-regulating motor.

• Strong self-priming capability, allowing liquid to be directly sucked in without a bottom valve.

• The pump is reversible; the liquid flow direction changes with the rotation direction of the pump, suitable for applications requiring repetitive pipeline flushing.

• Operates smoothly with low vibration and noise levels.

• Simple structure, facilitating ease of disassembly and repair.

Selection of Mud Pumps

1. Energy-saving
The selection of mud pumps is often greater than the actual requirements. It’s notable that the power of mud pumps equals the product of displacement and pressure as well as the ratio of displacement and flushing. While maintaining the same pressure, both flushing and displacement decrease, leading to diminished power consumption. During scenarios where pressures are below the rated pressure, the decrease in power consumption becomes even more significant. Therefore, setting the displacement according to the actual situation can significantly lower energy expenses.

2. Operational Stability
By eliminating high-fault transmission equipment, the motor rotor is composed of unmagnetized rare earth permanent magnet materials, ensuring stable and reliable operation with several grades of enhanced reliability over asynchronous or DC motors.

3. Safety
This machine incorporates a special frequency conversion drive with protection functions including overvoltage, overcurrent, overload, undervoltage, output grounding, and output short circuit, which maximally safeguards the motor, prevents combustion, and reduces losses caused by mud pump downtimes from power failure.

4. Reduced Maintenance
The motor directly drives the input shaft of the mud pump, eliminating intermediary transmission links found in traditional pump units, simplifying structure, conserving maintenance time for transmission components, and achieving a maintenance-free operation.

5. Reduced Equipment Volume and Weight
By removing associated components such as belts, pulleys, and covers, the structure is streamlined, resulting in a reduced pump unit volume and weight, thereby enhancing operational convenience.

6. Less Noise and Vibration
The direct drive of the mud pump decreases the use of belts or chains, significantly reducing vibration and noise, thereby improving the working environment for employees.