Automatic Control of Bulk Cement Tank Levels

Abstract

ABSTRACT The delivery of dry bulk cement to the cement mixer can be a difficult task. The level of cement in the surge tank must be controlled as the flow into and out of the surge tank changes. An automated system has been designed that is capable of maintaining a proper level in the surge tank. The system is primarily designed for use on offshore rigs and reduces the need for constant attention by an operator. This paper will cover:the bulk level control system requirements,the selection of components and,the operation of the system. INTRODUCTION In the past, steady flow separators on offshore rigs were operated by fairy available person on the rig. Typically, this was any rig crew member who was not busy during the cementing job. This meant that often the operator was untrained and, due to the turnover of personnel, rarely the same person twice. To determine the level of cement in the tank, the operator used the sight glasses which were often dust covered or tapped on the tank with a hammer to determine the level by a difference in sound. To overcome these operating problems, an automated bulk level control system was designed to control cement flow into the surge tank. The system monitors the level in the surge tank and adjusts the inlet flow as needed. SYSTEM REQUIREMENTS The normal practice of handling bulk cement is for the operator to watch a dust covered sight glass and attempt to keep a uniform Level of material in the steady flow separator based on this sight reading. Some rigs are equipped with a scale to weigh the steady flow separator (surge tank). This tells the operator the weight but not the level in the tank. Possibly 10%of rigs have scales. Using the scale, a lightweight cement blend can overfill while the same weight of heavier blend may result in the correct material level. Proper control of the material is not an easy job. Dust collects on the inside of the sight glass and makes the surge tank look full when it is empty. The light installed on the inside of the tank helps, but frequently burns out. Typically, the only reliable indication of the level in the tank is found by tapping the outside of the tank with a hammerand trying to listen to the difference in tone between full and empty at that location. Several undesirable events can happen with inexperienced operators. The surge tank could unintentionally empty. When dry cement stops flowing to the cement mixer, this causes a drop in the density of the cement slurry. Alternately, the surge tank could overfill. This could plug the vent piping with dry cement, making it difficult to remove the dry cement or the excess conveying air from the tank. The result is change in the density of the cement slurry.