Sandjet Tube Decoking
This technique is employed by Union Carbide Industrial Services Company (UCISCO) under contract. UCISCO has offices near major refining centers.
Steel shot, occasionally crushed flint, is blown through heater tubes by nitrogen carrier gas, physically chipping and scouring coke from the tube surface. The procedure is carefully monitored to avoid erosion of metal surface, particularly in return bends.
The procedure is faster and less costly than steam-air decoking, the usual alternative. It does not subject tubes to high temperature; therefore, it can be used on convection section tubes. Sandjet removes inorganic deposits. A disadvantage is that the technique works best on softer coke such as is formed in gas oil and atmospheric crude unit heaters. The method is used on crude vacuum heaters also, but the success record is not as good. Sandjet is seldom recommended for delayed coker heaters, where highest temperatures occur.
The method requires precise control of velocity. High velocity erodes tubes; low velocity decreases effectiveness of coke removal. Pass splits or changes of tube diameter partway through passes make sandjet decoking inappropriate.
Special Equipment Required
Sandjetting is carried out on one pass at a time. Manifolds are needed at inlet and outlet. UCISCO connects its equipment to the inlet manifold. The outlet connects to a covered container for collection of spent shot and coke. A typical procedure includes the following steps:
1. Remove meter orifice plates, thermowells, and other hardware subject to erosion.
2. The heater may be dry fired to 500° to 700°F to dry the coke and make it more friable.
3. Pressure drop of the coked tube pass is measured for a known gas flow. Thereafter, decoking progress is monitored by the reduction of pressure drop.
4. A decoking run takes 10 to 15 minutes and corresponds to the use of one charge of shot. Complete decoking may require up to 10 runs. Tubes may be inspected between runs for progress of coke removal and possible erosion of metal.