Comparision of Surge Systems

Comparison of Surge Systems

Item	Surge-Anticipating valve	Air vessel
Negative pressure prevention	Requires design of proper air valves and/or one-way filling tanks.	Very large capacity may be required for the pipe filling stage and local high points.
Accessories	Does not require any external additions except a drain pipe to the sump.	Requires compressor, backup compressor & sophisticated control systems to maintain the proper water volume in the tank
Check valve slam	Prevents mechanical slam to the check valve.	Increases mechanical slamming to the check valve.
System Cost	Low-cost for larger systems - valve size is reduced, mostly, as the pipe is longer. (longer pipes have a higher friction, and returning flow is smaller)	Very high cost for large systems. The tank size increases with capacity of the pipe, so longer pipes require larger vessels
	Expensive, not practical for small networks (up to 8")	Low cost for small system protection
Maintenance	Low-cost and easy maintenance - only periodical testing required.	High maintenance costs: Compressor, control system and micro-crack detection
Testing	Enables periodical operation tests  without stop of pumping.	Does not allow a test without completely stop of pumping.
Footprint	Requires minimal space	Requires large space
Relief function	High Pressure Relief function is added as a standard feature of the valve	Not included, cannot prevent abnormal high pressure in the network during pumping
Protection of raw-water & sewage system	Included in the standard valve. Same protection. No additional parts. Clean or filtered media is required.	Makes the maintenance even more difficult & expensive task. Not recommended.

Summary

Though air-vessels are the most common method of surge-protection used nowadays in design of pumping system, it is our opinion that using Surge-Anticipating Valves should be seriously  considered due to its low cost, ease of maintenance (cost of ownership) and reliability.

Combination of Air Release Valves with Surge-Anticipating valves, if properly sized and adjusted, will supply an equal level of protection for large systems, at a much lower cost and smaller footprint.

Pipe Stress- Hoop, Longitudinal & Radial

Induced Stresses in Pipe- Hoop, Longitudinal and Radial Stresses

Introduction to Induced Stresses:

When the ends of the pipe ends are closed and pipe is subjected to an internal pressure ‘P’ there are various stresses that develop in the pipe. Each element of pipe are subjected to the below mentioned stresses which act in the direction as shown in the fig.1.

• Circumferential (hoop) stress sH
• Longitudinal Stress sL
• Radial Stress sR

Fig 1: Different stresses induced in pipe

CIRCUMFERENTIAL OR HOOP STRESS: sH

The effect of this may split the pipe into two halves as shown in fig.2. The failure of the pipe in two halves in fact is possible across any plane, which contains diameter and axis of the pipe. Elements resisting this type of failure would be subjected to stress and direction of this stress is along the circumference. Hence the above stress is called Circumferential or Hoop Stress.

If -

D = Diameter of the pipe

L = Length of the pipe

t = thickness of the pipe.

Then

Bursting force, FB	=	Pressure * Area
	=	P * D * L
Resisting force, FR	=	Resisting metal area * Stress, sH
Equating FB & FR
P * D * L	=	2t *  L *  sH
∴ t	=	(P * D)/ 2 * sH
or         sH	=	(P * D) /( 2 * t)    ______________________(1)

This equation is used for calculating the thickness of pipe so as to withstand pressure ‘P’ where s H is allowable circumferential stress.

Fig 2: Circumferential or Hoop stress

LONGITUDINAL STRESS:sR

Considering that the pipe ends are closed and pipe is subjected to an internal pressure ‘P’ the pipe may fail as shown in Fig.3. Elements resisting this type of failure would be subjected to stress and direction of this stress is parallel to the longitudinal direction of the pipe. Hence this stress is called longitudinal stress.

Then

Bursting force, FB	=	Pressure × Area
	=	P * (πD * D)/4
Resisting force, FR	=	Resisting metal area x Stress, sL
	=	π D t * sL (when t is significantly small as compared to D)
Equating FB & FR
P * (πD * D) /4	=	π D t * sL
∴ t	=	(P D)/4 * sL
or  sL	=	(P D)/(4 * t)       _________________________ (2)

NOTES: 

1)     On comparing equations 1 & 2, it is clear that when a pipe having diameter ‘D’ and thickness ‘t’ is subjected to an internal pressure ‘P’, the induced circumferential tress is double the induced longitudinal stress.

2)     Normally, the pipe is considered as a thin wall cylinder i.e. t < D/6 

3)     Usually D is substituted by Do (outside diameter) in order to have higher safely margin.

Fig 3: Longitudinal stress

 RADIAL STRESS: sR

Radial stress is a stress in directions coplanar with but perpendicular to the symmetry axis.

The radial stress for a thick-walled pipe is equal and opposite to the gauge pressure on the inside surface, and zero on the outside surface.

The radial stress is always compressive.

Each element of the pipe is subjected to radial stress which acts in radial direction as shown in Fig.4 and calculated as

sR                           =          P

Fig 4: Radial stress

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Source: http://www.piping-engineering.com