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Mixing Head, Cement Metering Valve, & Diffuser

Objectives

  • Recognize the mixing head, cement metering valve and diffuser.
  • Explain the function of the mixing head, cement metering valve and the diffuser.
  • Explain the importance of the mixing head, cement metering valve and diffuser in the cementing process.

A Word About Slurry

The cement densities that you are given for a job are very specific. The blends and densities requested are chosen carefully and are the result of a lot of laboratory testing.

For your cement to perform as expected, the batch that you field mix must be as close as possible to the slurry mixed in the laboratory tests.

This means mixing slurry of uniform consistency at the correct density is critical to every cementing job.

When Good Slurry Goes Bad

There are several potential problems with field mixing large batches of cement.
  • Clumping
  • Air Entrainment (Air particles trapped in the mix)
  • Dust
  • Dry Blending

Clumping causes spikes in density

Clumping

The dry cement powder must be wetted evenly on its first pass through the mixing head or the unwetted powder particles will clump together.

If there is clumping in the slurry, even if it’s not visible to the eye, the slurry density has been compromised.

These clumps are tough to break up even if the slurry is recirculated through the mixer.

Air Entrainment

Air entrainment doesn’t overly affect the strength of the slurry blend, but it will reduce the accuracy of your density measurement.

Some air will always get trapped in the slurry as part of the mixing process, but the less, the better.

Cement Powder + Lungs = Bad

Dust

There's nothing good about raising a cloud of dust during mixing.

  • If a lot of cement powder is escaping as dust, it’s not going into the slurry blend, so the slurry might not be getting mixed to the correct specification.
  • When pumping to the cement unit or when aerating (adding air to) the powder before delivery, the bulk operator is required to wear a respirator. If there's dust in the air, there is a chance you're breathing it in.
  • If there is a lot of dust, it also interferes with your vision of the mixing tub, which makes correct mixing much more difficult.

Dry Blending

Dry blending occurs when the ratio of mix water to cement powder is too low. The result is an unusually thick and dry slurry that will not flow through the ports in the bottom of the diffuser and into the mixing tub.

The diffuser will quickly plug up and fill to the top. Cement dust and even slurry may be pouring from the diffuser air vent at this time. This is a considerable mess to clean up with cement powder wasted and valuable pumping time lost.

Dry blending can occur while building up density under manual control. The operator must be careful to start the mixing with an ample rate of mix water before introducing cement powder.

One of the easiest ways to guard against dry blending is to keep the cement metering valve at less than 50% open. This may limit the speed at which you can cement, but if you mix too fast, you won't be able to recover if you start to dry blend.

Keeping Slurry on the Straight & Narrow

The mixing head, cement metering valve, and the diffuser work together to reduce powder clumps in the slurry, air bubbles and escaping dust.

The Mixing Head

The mixing head’s purpose is to reduce/eliminate clumping by fully wetting all of the cement particles on their first pass through.

It’s vital that the mixing head is cleaned thoroughly after each job.

How It Works

The mixing head has several ports for water, recirculating slurry, and bulk cement powder. The water and recirculated slurry enter the mixing head at high pressure. The head is shaped to combine these flows in a vacuum vortex. This creates an evenly mixed slurry blend which leaves the mixing head at high speed.

An advantage of the newer mixing head styles is their “straight through” design; there are no parts sticking out into the chamber to complicate washout and maintenance.

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