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When need repair the storage tank. first to unscrew the hand wheel (use to fixed manhole cover and supply the force to axial seal), then open the manhole cover by the handle whick fixed in the manhole cover ( manhole cover is jointed together by hinge and frame, to ensure accurately closure when the manhole cover close, and the hinge can let opened manhole cover stop above 30 degree angle of upright position), thus the serviceman can check the storage tank; when the Manhole cover closed, both of the seal of manhole cover and the tank can ensure complete sealing, so, we can avoid leakage of the tank.