criteria for surge analysis

Question

Dear: 
 kindly, it is required to study the follwing criteria for surge analysis in Sudia Arabia project; 
 To verify the surge vessels volume, two relevant critical cases, as part of the transient analysis, shall be considered: 
 1)Pump power failure assuming that the air volume is -5% in combination with n=&kappa;=1.4 (isentropic condition is most critical regarding minimum pressure along the pipeline).? for this requirement; i dont understand what exactly meant by the air volume is -5%, is it the air voulme in the vessel 5% of the total volume. in addition, what is the purpose for the -ve sign. furthermore, the n vaule = 1.4 is it for the exponent of the gas law? 
 Pump power failure assuming that the air volume is considered to be +5% in combination with n=1.0, (isotherm condition is most critical regarding blow out of the surge vessel). for this requirement; i dont understand what exactly meant by the air volume is +5%, is it the air voulme in the vessel 5% of the total volume. in addition, what is the purpose for the +ve sign. 
 many thanks

Jesse Dringoli · Answer

Hello Sawsan, 
 Re: "what exactly meant by the air volume is -5%" 
 If it is referring to a hydropneumatic tank, maybe it means that you should simulate the tank with 5% less air volume than normal, but you'd need to know what that baseline normal air volume is. It may be best to ask the reviewing agency (or whomever provided the requirements) for clarification. 
 Re: " the n vaule = 1.4 is it for the exponent of the gas law? " 
 Yes, that seems to be referring to the exponent in the gas law. For a transient analysis in HAMMER, it is hard-coded as 1.4 for air valve calculations, but can be changed for hydropneumatic tank calculations - see "gas law exponent" property for a hydropneumatic tank node. More on this can be found in this article: Modeling Reference - Hydropneumatic Tanks 
 Re: " what exactly meant by the air volume is +5% " 
 My comment on this one is the same as -5% - I would suggest confirming with whomever established the requirements. Maybe in this case it means you should simulate the initial gas volume 5% greater than normal.