Prior to the use of centrifugal pumps on drilling rigs, the standby reciprocating mud pumps were customarily used to operate the mud hopper. As with high pressure mud guns, this required high-pressure pipe and connections. This was costly because the pump required enormous power and expensive piping. A small orifice in the hopper delivered a low flow at a high velocity. The jet velocity was suitable for adequate mixing, but the volume was usually less than 500 gpm. This, of course, would limit the speed of material addition.
Since the 1950s the centrifugal pump has been the predominant tool for charging mud hoppers on drilling rigs. This permits the use of lowpressure equipment and the movement of large volumes of fluid rapidly. Savings were realized through reduced piping cost and higher addition rates that lowered operating cost. It also released the standby main pump from mud mixing duty.
A low-pressure mud hopper is shown in Figure 1. Since highpressure hoppers work in the same manner, this discussion will be limited to low-pressure equipment. The fluid velocity in a low-pressure mud hopper will be around 10 ft/sec on the pressure side of the jet nozzle. The pressure line is reduced in size usually from 6 to 2 inches (152 to 51 mm) and exits the jet nozzle at a much higher velocity but lower pressure.
The high-velocity jet stream crosses the gap between the eductor nozzle and the downstream venturi and creates a partial vacuum within the mixing chamber (or tee). This low-pressure area within the tee, along with gravity, actually draws mud materials from the hopper into the tee and fluid stream. The high-velocity fluid wets and disperses the mud additives into the fluid stream. This reduces lumping of material and is an initial shear of the additives.
Most mud hoppers use a venturi for two reasons: (1) to increase the shearing action of the mud and (2) to help regain some of the pressure head to move the mud downstream or upward as it returns to the mud system.
The venturi reduces the time to build viscosity when bentonite is added. This is due to the increased shearing of the fluid that takes place within a well-designed venturi. Similar results have been observed with other drilling fluid additives.
At point A in Figure 10.19, the pressure head is high, while the velocity head would be low, as in the previous examples. Assume that the system is level, so that the elevation head is be zero. As the fluid moves downstream through the venturi, the total head at point C would theoretically remain the same, with the velocity and pressure head being equivalent to that at point A. As in the previous examples, the main problem with this equation is that it ignores friction head losses that in practical applications can be about 50%.
In actual practice, friction head must be accounted for, and if the venturi were not present, there would be tremendous turbulence as the flow expanded into the larger-diameter pipe. The venturi simply helps to streamline the flow back to the large pipe with less turbulence. This results in a minimum friction loss and will provide the maximum available head to push the fluid and newly added commercial additives from the hopper into the piping system.
Mud hoppers come with valves to isolate the hopper from the mixing chamber. When closed, the space between the venturi chamber and the valve is less than atmospheric, i.e., a partial vacuum is formed. The amount of vacuum at point B influences the resultant addition rates of the device and is determined by several factors, including feed pressure, nozzle diameter and length, venturi design, fluid properties, and downstream piping restrictions.
One manufacturer modifies this concept by installing a premix wetting chamber between the hopper and the eductor and also by modifying the eductor. A tee installed upstream of the eductor diverts part of the fluid into the premix chamber, where it swirls and forms a vortex that radiates outwardly to the wall of the premix chamber. The lower pressure at the vortex center draws material into the center as well as downward and into the eductor. The eductor is also different in that it features a starshaped cross-sectional feed area, as opposed to a circular nozzle. The advertised benefits are the swirling action and more thorough mixing
Mud Hopper Installation and Operation
As with any piece of equipment, the mud hopper and related equipment must be sized, adjusted, and installed properly to achieve optimum performance.
Once a mud hopper has been selected, the pump, motor, feed line, and discharge line must be designed to allow the proper flow rate at the recommended head. For example, assume that a mud hopper has been selected that requires 550 gpm (2082 lpm) at 75 feet of head (23 m). With this established, feed- and discharge-line sizes would be determined by recommendations from a recognized authoritative source.
The friction head would be determined for all lines and connections at 550 gpm and this would be added to the 75 feet of head for the hopper. The next step is to select a pump and impeller that will provide 550 gpm at the total head required. With the pump selected, the motor size can be determined and adjusted for mud weight. If the maximum mud weight to be used is unknown, it is standard practice in most companies to base all calculations on 20 lb/gal mud (2.4 SG).
Feed and discharge lines of the mud hopper should be kept as short as possible. This is dictated by economics (i.e., less power, piping, and smaller pump) as well as operations. The backpressure from the downstream piping is crucial to effective operations. Figure 1. shows the effect of system backpressure on mixing chamber pressure.
When using a 2-inch (51-mm) jet nozzle with a 3(1⁄8)-inch (79-mm) gap, there is a very strong vacuum at normal operating pressures of 70 to 75 feet of head until the backpressure reaches 50% of the inlet pressure, at which point mud will actually try to backflow through the mud hopper. Obviously, the downstream pressure drop must be less than 50% of the inlet pressure, and systems should be sized accordingly.
System backpressure also has an effect on the rate at which materials can be added to the system. It should be noted that the vacuum was about 25 inches of mercury (0.85 bar) when a venturi was utilized. When the venturi was removed, the vacuum was measured at only 9(1⁄2) inches Hg(0.32 bar). A small change in the backpressure causes a measurable change in the addition rate. An even more noticeable effect on the addition rate is the relationship between the gap between the jet nozzle and the venturi.
When the gap is between 1 and 3(1⁄8) inches (25.4 to 79.4 mm) for this particular mud hopper, there is very little difference in barite addition rate. When the gap is larger than 3(1⁄8) inches, the rate of barite addition is reduced significantly. This further illustrates the need to have equipment and piping sized and adjusted properly.
Another important consideration when installing addition equipment is the amount of lift required. If the discharge goes to a tank on a different deck or there are tall tanks and the hopper is on the ground, mixing rates can be reduced. Sack barite addition is reduced by 17% when the lift is increased from 6 feet to 12 feet (1.83 to 3.66 m). If the lift requirements are severe and the pump is undersized, the mixing capabilities will suffer.
From the preceding discussion, it is obvious that the mud hopper can be highly efficient, while improper mud hopper installations can create many problems. Low addition rates lead to increased rig time and operator hours to treat the mud system.
If the discharge from the mud hopper is routed into a tank that is not stirred properly, a large quantity of commercial materials could settle, even if the hopper is properly dispersing and wetting the materials. If the addition system is installed, sized, and adjusted properly, there will be a reduction in system maintenance, a decrease in fluid costs, and a decrease in operator hours.
Mud Hopper Recommendations
The following recommendations will promote efficient mud hopper installation and use:
- Select a mud hopper that is properly sized for the mud system. Generally, a single hopper is sufficient for most rigs. If the mud circulating rate is greater than 1200 gpm (4550 lpm), then consider using a hopper with 1200-gpm capacity. Generally, there is no need to add chemicals faster than this. For many operations, 600 to 800 gpm (2270 to 3030 lpm) is adequate.
- Keep the lines to and from the hopper as short and straight as possible. Size the pump and motor based on the system head and flow rate requirements. A venturi is beneficial in all operations, but especially when the system backpressure may reduce the mud hopper efficiency and operation. The venturi will allow fluid to move vertically higher than the hopper height. On many rigs, hoppers are placed at ground level and the downstream pipe is raised to a height equal to or greater than the top of the mud tanks.
- Use new or clean fittings to reduce friction loss. After each operation, flush the entire system with clean fluid to prevent the mud from drying and plugging the system. Clean the throat of the hopper to prevent material from bridging over that will cause poor performance the next time the hopper is used.
- A table should be attached to or located near the hopper to hold sacks of material. The table should be at a convenient height (36 to 42 in., or 0.9 to 1.1 m) so that personnel can add material easily with minimal strain. Power-assisted pallet and sack handlers will enhance addition rates and minimize personnel fatigue.
- As with all equipment on the rig, develop a regular maintenance and
inspection program for the mud hopper. The mud hopper is normally
simple and easy to operate, but worn jets and valves will hinder the
operation. Inspect the entire system every 30 to 60 days. Maintain an
inventory of spare nozzles, valves, and bushings.
If the jet action and dispersion appear to be substandard, check the
- Determine that the pump is providing an adequate volume of mud at the normal operating discharge head. A gauge upstream of the hopper will verify the pressure or head. A reduced pump discharge volume is normally caused by air entering the air pump packing, an object lodged in the piping, a worn impeller, gas or air causing the pump to airlock, a connection or piping leak, or dry mud packed around the jet and restricting the mixing area within the throat.
- With the pump shut down, remove the hopper and valve from the tee.
Unions installed upstream from the hopper allow disassembly as well as inspection to determine whether the inside passage is eroded or an object is lodged in the eductor.
Other Shearing Devices
Like the mud hopper, a number of devices are available to increase
shear, speed hydration, and enhance curing and saturation. Prehydrating
has several advantages that it:
. improves the efficiency of addition, thus reducing over- or undertreatment. Overtreating may force dilution and the possible removal of excess fluid to make room for the increased fluid volume. Undertreating will hinder fluid performance;
. reduces water loss to the formation by enhancing filter cake properties;
.prevents formation of unblended globular masses, or ‘‘fish-eyes,’’ in polymers;
. prevents chaining or stringing of polymers;
. stabilizes properties faster for more accurate checks; and builds saturated salt solutions, pill, or slug volumes quickly.
Other shearing devices can be as simple as a small tank (less than 50 bbl, 8 m3) with a centrifugal pump and nozzles to circulate the fluid, thus exposing it to high shear in a short period. Many systems enhance centrifugal pump performance with a specialized fluid-end design that features a plate, porting, and multiple internal nozzles (Figure 10.22).
These have proven effective by producing up to 83 ft/sec (25.4 m/sec) of mechanical shearing velocity within the impeller housing and 100 ft/sec (30.5 m/sec) of liquid velocity. The close particulate proximity and fluid channeling prevent large particles from exiting the pump without first breaking into much smaller particles. These devices are suitable for enhanced shearing and reducing curing times, and they are ideal for prehydrating additives prior to blending with the active system.