Giá trị này có thể được đại diện về di dời (mils), vận tốc (cm mỗi giây), hoặc tăng tốc (inch trên giây bình phương), trong số đó là thảo luận chi tiết hơn trong phần đo lường tối đa độ rung mà sau. Biên độ có thể được đo như Công nghệ hạn chế giới hạn vấn đề bảo trì ảnh hưởng của hạn chế các biến quá trình đào tạo Thông Phiên điều hành năng động | 222 An Introduction to Predictive Maintenance which either increases or decreases depending on the amount of work the pump must perform. Flowrate. The volume of liquid delivered by the pump varies with changes in TSH. An increase in the total system back-pressure results in decreased flow whereas a back-pressure reduction increases the pump s output. Correcting Problems The best solution to problems caused by TSH variations is to prevent the variations. Although it is not possible to completely eliminate them the operating practices for centrifugal pumps should limit operation to an acceptable range of system demand for flow and pressure. If system demand exceeds the pump s capabilities it may be necessary to change the pump the system requirements or both. In many applications the pump is either too small or too large. In these instances it is necessary to replace the pump with one that is properly sized. For applications where the TSH is too low and the pump is operating in run-out condition . maximum flow and minimum discharge pressure the system demand can be corrected by restricting the discharge flow of the pump. This approach called false head changes the system s head by partially closing a discharge valve to increase the back-pressure on the pump. Because the pump must follow it s hydraulic curve this forces the pump s performance back toward its BEP. When the TSH is too great there are two options replace the pump or lower the system s back-pressure by eliminating line resistance caused by elbows extra valves and so on. Positive-Displacement Pumps Positive-displacement pumps are more tolerant to variations in system demands and pressures than are centrifugal pumps however they are still subject to a variety of common failure modes caused directly or indirectly by the process. Rotary-Type Rotary-type positive-displacement pumps share many common failure modes with centrifugal pumps. Both types of pumps are subject to process-induced failures .
