a system for pumping a drilling fluid mixture , the system , in certain aspects , having a pump apparatus , the pumping apparatus having a body with an inlet and an outlet , a suction valve and a discharge valve in the body for selectively controlling flow through the body , or each valve having a curved valve seat and a valve member with a curved seating surface , the valve member selectively movable to seat against the curved valve seat to prevent the flow of the drilling fluid mixture past the valve seat ; and , in one aspect , a seal in the curved valve seat against which the valve member is also sealable . this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims , 37 c . f . r . 1 . 72 .