High Energy Mixing: necessary for quality mixing

Introduction

The mixing of slurry may seem to be a simple task.  However after considering the difficulty of mixing 3,000 lb (1360.77 kg) of cement with water in one minute, one can visualize the size of the problem.  The problem also is not all what it appears to be.  What seems to be well-mixed slurry may not be well mixed.  A poorly mixed slurry may have small or large lumps of dry cement in the mixing tank and may contain excessive entrained air.

 The problem with dry cement in the tank is obvious – not all the cement is being incorporated.  A poorly mixed slurry will contain clusters of dry particles that may not be visible to the eye.  Once these clusters exist, they are hard to break up.  Without all particles being wetted, then the slurry will not have the anticipated properties.  Compressive strength will be slower to develop, viscosity will be lower, fluid loss will be higher and free water is likely to be greater.

The process normally incorporates some air.  A poorly mixed slurry will contain more air due to air associated with the dry particles.  Also some mixers characteristically entrain more air.  Air in itself is not bad but can cause density measurement problems and may be indicative of low wetting efficiency. 

Slurry designs are tested in the laboratory before the field blend is blended.  The laboratory procedures used have been developed to obtain consistent and predictable results. Field results will yield similar slurry properties assuming that the materials used by the lab are representative and the mixing energy used by the field is approximately the same.  The mixing energy imparted by the laboratory equipment will yield well-mixed slurry.  This amount of energy also will yield properties that are stable (further mixing, to a point, will not cause slurry properties to change appreciably).  If half the required mixing energy is used, the slurry properties will not be fully developed and are dependent on the amount of energy.  Thus, mixing energy that is less than required will result in slurry properties that are not as predictable.

Mixing Energy

Field mixed slurries with inadequate mixing energy will not yield the same slurry properties as that which was mixed in the laboratory.  Thus, due to poor mixing, the slurry will not be the same as predicted and thus could cause problems.

Back to the problem at hand; let us examine how to mix 3,000 lbs (1360.77 kg) of cement per minute.  It is more difficult to wet clusters of dry cement particles after the cluster has formed than if the particles were fully wetted when they were in first exposed to water.  Thus the ideal process mixer is one that will fully wet all particles during the first pass through the mixer. 

With 3,000 lb (1360.77 kg) per minute of cement, this can only be accomplished with a high-energy mixer.  This means that the mixer must have adequate horsepower (should result  in at least 60% of API mixing energy).  In terms of engine horsepower, this is equivalent to about 30 HP/bpm of throughput.   If a 6 bpm mixing rate is desired, then 180 HP (134.22 kilowatt) is the minimum required.  Most mixers have a fixed amount of power available and therefore as the process throughput rate increases, the energy available per barrel of slurry decreases.  Some slurries are more difficult to mix than others and may require higher than 60% API mixing energy.  Slurries that have a low mix water requirement, require low to ‘0” free water, contain high quantities of bentonite and saturated salt tend to be harder to mix.  Thixotropic slurries with guar gel type thickening additives are also difficult. 

The following figure illustrates the expected %API mixing energy as a function of mixing rate.

Serva Mixing Systems were designed for high energy mixing and ease of operation and maintenance.  There are many of this family of similar mixing systems working throughout the world.  The applications have varied from onshore to offshore and dessert heat to cold climates. 

The mixing system consists of the following components:

• High-energy mixer
• Cementing metering valve
• Mixing tub
• Recirculating centrifugal pump
• Mix water centrifugal pump
• Agitating system
• Air separator
• Transfer pump (optional for single pump cementers and required for double pump cementers)

High-Energy Mixer

The slurry mixer is a recirculating type mixer designed to meet the most demanding mixing requirements.  The mixer design provides easy cleanup and maintenance. The mixer has a 5-1/2” union thread half on the inlet for attaching the dry bulk cement metering valve.  In the center of the mixing chamber is a high-pressure jet that wets the bulk cement from the center of the mixing chamber.  The recirculated slurry enters the mixing chamber from an outer annular position surrounding the dry bulk cement. The force of the recirculating slurry is directed inward and the force of the central jet directed outward creates the energy to mix the dry bulk cement. This provides maximum wetting efficiency.

 

The operator can easily adjust the water flow from almost zero to maximum. The valve is linear, providing equal increases in water flow per increment of displacement. Maximum water flow is a function of supply pressure. The recirculating mixer design improves mixing and homogenizing of the new incoming slurry with the slurry already in the mixing tank.

Bulk Cement Metering Valve

The rotary operated cement-metering valve adjusts the rate of bulk cement flow. It may be operated manually or hydraulically. The valve is approximately linear and will provide near equal increases in bulk cement delivery per increment of rotation. A particularly important feature is the ability to meter cement at low flow rates without bridging off (the phenomenon where cement stacks against the opening and therefore prevents further passage of cement).