In last article, 6 advantages between direct circulation and reverse circulation drilling is explained from application point of view, here will be some key point for how to design the reverse circulation drilling system.

There are total 7 points the designer should care to get a perfect RC drilling system

1. How to Define Sink Ratio?














The sink ratio which related to depth of air water mixer sinking under water is key to whole reverse circulation drilling system’s working efficiency

Below formula is how to calculate the sink ratio:

m = h1/(h1+h2)

m= Sink ratio

h1= height from water surface to outlet tube

h2= height from position of air water mixer to water surface

In General, m >=0.5, when m<0.5, the efficiency of fluid flowing up is lower, even fluid not flowing up at all.

For reverse circulation drilling system, the bigger of sink ratio (m), the more powerful of driving force, which can overcome the difficulties during drilling, such as collapse of well wall, leakage, increasing of resistance etc. Meanwhile, it can achieve high drilling efficiency.

For the depth of air water mixer in water, it can be fixed when the sink ratio is decided.

2. What is Air Pressure Value?

The borehole depth is decided by  rotary torque, lifting force and air pressure which is one of key factor. Due to the principle of gravity, 10 m high water column is equal to 1 bar, 10 m = 1 bar = 0.1 Mpa.

Take water well drilling as example, for 400 m depth borehole  with 200 m length dual wall pipe, the air compressor with 20 bar output pressure is ok in theory. But considering the back pressure and leave some buffer as well from safety point of view, we should select the air compressor with 25 bar output pressure

3. What is recommended Discharge Rate/Air Speed?

Recommended discharge rate is 20 – 33 m/s, the lowest rate can not be less than 15 m/s. if the air speed is too high, it will make too much dust and damage the collar of hole. If the speed is too low, it will not bring the cutting out of hole timely, which will lower the drilling efficiency even stuck drilling.

The drilling efficiency and dust level are most important for foundation drilling and mining drilling, due to normally the hundreds or thousands of boreholes needs to completed in given period.

4. How to Define Air Flow?


BV = Bailing Velocity = m/min

AV = Air Volume = m3/min

AA = Area of Annulus

Based on the formula above and the recommended discharge rate, the required air flow can be gained. When the air pressure and air flow is confirmed, the needed air compressor can be selected easily.

The proper compressor is key for hydraulic drilling rig which fit with foundation drilling and water well drilling as well as mining drilling for rock formation. See article  Rock Properties and Rock Drillability

5. What is The Total Length of Dual Wall Pipe?

The length of dual wall tube is in direct proportion to hole depth, The recommended dual wall tube length is one third to half of hole depth

6. How to Define Diameter of Dual Wall Tube?

It is better to get bigger diameter for dual wall pipe if possible, the basic request is tube dia. > 1/10 of hole dia.

7. What is the Best Borehole Depth?

The reverse circulation drilling can not be used for borehole depth less than 1o m and normally it is used after 50 m depth for well drilling

Above is just Hardrock‘s general summary based on our engineers many year’s experiences on borehole drilling area, for any specific application, it has to be considered specifically.

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