Introduction of Mud Gun

Both high-pressure and low-pressure mud guns agitate mud by means
of rapid fluid movement through a nozzle. Pressure and flow are delivered
via either high-pressure main mud pumps or, most often, centrifugal
pumps. There are two points of view as to where pumps for mud
guns should draw suction from:

(1) the compartment that they stir or
(2) a compartment downstream, ideally immediately after the last solids control device.

Consider option 1 (drawing suction from the compartment to be
stirred): When mud guns are placed in compartments that are designated
for solids-removal equipment (e.g., hydrocyclones, centrifuges), the possibility of pumping solids-laden fluid exists. Continuous circulation of solids-laden fluid (especially sand) will result in faster wear on pump parts and mud gun nozzles. Manufacturers provide nozzle inserts made of wear-resistant plastics to increase life and reduce replacement costs.
If parts are not replaced when needed, agitation efficiency decreases.
Another argument against pumping into the same compartment from
which suction is drawn is the possibility for particle size degradation,
which will also reduce solids-removal efficiency and force use of more
costly solids-control mechanisms or dilution. Option 1 is feasible if used
in compartments downstream of the last solids-control device, provided
the upstream solids-control system is properly designed and operated.

Option 2 (drawing suction from a compartment downstream of the
solids-control system) is more desirable for two reasons. First, it prevents
continuous circulation of fluid that may be laden with drilled solids,
assuming efficient upstream solids removal. This maintains mud gun
efficiency and reduces wear to fluid-end pump parts. Secondly, it places
cleaned fluid into the compartment to be stirred. This is especially
effective when solids concentration is high or fluctuates (e.g., high
penetration rates and/or sweep returns to the surface). However, option
2 necessitates more process capacity from the solids-control equipment
(including the degasser and hydrocyclone compartments). Rig circulating
rate and flow through mud guns must be added and then accounted
for. Particular examples are discussed later in this chapter.

High-Pressure Mud Guns

High-pressure guns typically come in 3000 and 6000 psi ratings and
require heavy-walled piping. Gun nozzle sizes range from 1⁄4 to 3⁄4 inch (6.4to 18.4 mm). The rig’s main mud pumps (positive displacement piston types) pressurize the guns. The high-pressure system requires heavy piping and connections but relatively small nozzles. It is a high-pressure, low-volume system. The agitation is a result of high-velocity fluid coming from the jet nozzle.

Low-Pressure Mud Guns

Low-pressure mud guns usually require about 75 feet of head for effective
operation (see chapter 18 for head and pump sizing). Nozzle sizes
range from 1⁄2 to 1 inch (12.7 to 25.4 mm). Centrifugal pumps pressurize the nozzles through standard wall piping (typically schedule 40 pipe is used). The low-pressure system does not require heavy-walled piping.
Because of higher flow rates, larger-diameter pipe is used to prevent
excess friction loss. The jet nozzles are larger than in the high-pressure
system. Effective agitation occurs from the large volume of fluid entering
the mud tank through the nozzle. Fluid shear is applied by the velocity of
the fluid exiting the nozzle. This is called a high-volume, low-pressure
system.

With either type of system, it is economically as well as functionally
desirable to keep the jet nozzle feed lines as short and straight as
possible. With a low-pressure system, this is not merely desirable, but
critical for efficient operation.

Eductors

Eductors (also called jets) are used on some systems and can achieve up
to four times the fluid movement over conventional nozzles. Eductors
create a low-pressure area around the discharge of the nozzle that draws
in and entrains fluid immediately around the eductor. This is similar to
the action in a mixing hopper. The two fluids are mixed in the venturi
section and discharged into the tank at a much higher rate than the jet
nozzle alone could achieve. The high-velocity fluid from the jet can either
be the same fluid in the tank or be from elsewhere (e.g., mixing and
blending compartments).

Mud Gun Placement

Mud guns are usually placed about 6 inches (15 cm) from the tank
bottom and typically come with a 360 swivel that allows directional
positioning to stir dead spots. Dead spots can occur in right-angle compartments that have inadequate mechanical agitation or can be caused
by piping or other mechanical obstructions.

It is generally accepted that low-pressure nozzles are effective within
a 5- to 9-feet diameter depending on the mud weight, viscosity, and
nozzle velocity. Nozzle size and feed pressure determine how much fluid
will pass through the nozzle and at what velocity. Greater volumes of
fluid through the nozzle produce more circulation of fluid, while higher
velocity creates more shear. Bernoulli’s theorem describes the relationship
of pressure and velocity. High-pressure fluid exits a jet nozzle at
a high velocity, where head is converted to velocity. One manufacturer uses nozzles that are housed within an eductor body that also includes multiple induction ports. The eductor body acts as a mixing chamber and diffuser to induce a swirling flow out of the body. Fluid is pumped into the central nozzle, which then draws fluid through the side induction ports. The manufacturer states that the use of this type of nozzle will increase the amount of fluid moved by up to four times what is delivered to the orifice. An example of how this would be applied is shown in Figure 1

Multiple eductors mounted on static pipe.
Figure 1. Multiple eductors mounted on static pipe.

Sizing Mud Gun Systems

Since most systems employ low-pressure mud guns, it is critical to ensure
that the proper centrifugal pump and piping are chosen. A great deal of
the literature has been devoted to sizing pumps and will not be duplicated
here (see chapter xx for full information). It must be emphasized
that for a low-pressure system to work properly, several items must be
considered, including:

. Jet nozzle size and number
. Number of turns, tees, valves, and reducers
. Pipe length
. Pipe sizes used

All are important, and the effect of each must be determined before
a pump can be selected. In most systems, the pressure drop across the
nozzles will consume most of the total head delivered by the pump, but
discharge line length, pipe diameter, and fittings must be considered to
avoid oversizing or undersizing a pump and motor.

To achieve a given flow rate through a jet nozzle, a specific total head
will be required to overcome friction through the system. Figure 2
shows the discharge capacity (flow rates) for various sizes of ideal nozzles
at several heads. If reducers or swages are used, the capacity will be
lower. The longer the piping, the greater the number of fittings; and/or
the smaller the diameter of the piping, the greater the total head
required. General rules are:

. Fluid velocity should be maintained between 5 (to avoid solids settling)
and 10 ft/sec (to avoid excessive pipe erosion) (1.5 to 3 m/sec).
. Fittings and connections should be minimized.
. Piping should be kept as short as possible.

Flow rates through ideal nozzles
Figure 2 Flow rates through ideal nozzles

All are important, and the effect of each must be determined before
a pump can be selected. In most systems, the pressure drop across the
nozzles will consume most of the total head delivered by the pump, but
discharge line length, pipe diameter, and fittings must be considered to
avoid oversizing or undersizing a pump and motor.

To achieve a given flow rate through a jet nozzle, a specific total head
will be required to overcome friction through the system. Figure 10.14
shows the discharge capacity (flow rates) for various sizes of ideal nozzles
at several heads. If reducers or swages are used, the capacity will be
lower. The longer the piping, the greater the number of fittings; and/or
the smaller the diameter of the piping, the greater the total head
required. General rules are:

. Fluid velocity should be maintained between 5 (to avoid solids settling)
  and 10 ft/sec (to avoid excessive pipe erosion) (1.5 to 3 m/sec).
. Fittings and connections should be minimized.
. Piping should be kept as short as possible.

It is important to consider total system efficiency, by accounting for the
added flow into the compartment, when using mud guns in the degasser
or hydrocyclone compartments if the mud gun supply comes from elsewhere in the surface system. The volume fraction treated can be calculated by determining the equipment capacity, for example, for a desilter, and the circulating rate of the fluid entering the desilter suction compartment.
As a rule, hydrocyclone and degasser systems should be sized to
process between 110 and 125% of the maximum flow volume through
their compartments. For example, if the rig’s circulating rate is 600 gpm,
the desilter capacity is 1000 gpm, and the mud gun rate (with suction from a downstream compartment) is 200 gpm, then 1000/[200þ600]¼ (1000/ 800), or 10⁄8 (125%) process efficiency. If the desilter capacity is 700 gpm, then 700/[200þ600]¼(700/800), or only 7⁄8 process efficiency. In this situation, an undesirable 87.5% process efficiency is achieved. To counteract this inefficiency and achieve at least 100% process efficiency, the desilter capacity must be increased to greater than 800 gpm or the mud gun rate reduced to less than 100 gpm. A combination of the two technologies yields more desirable process efficiency, that is, greater than 100%.

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