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TL-161

Figure 1. Overview of compact

odour-scrubbing installation showing the

six Vanton ANSI end-suction

polypropylene pumps handling acid and

alkaline solutions over the pH range

from 3 to 12 at 650 gpm against a 38'

TDH.

Figure 2. Two Vanton polypropylene sump

pumps handling caustic soda and chlorine

solutions.

Pump Material Selection

Guide: NaOH Sodium

Hydroxide/NaOCl Sodium

Hypochlorite

INDUSTRY:

ENTITY:

SOLUTION(S) PUMPED:

PUMP TYPE(S):

Chemical, Metal Finishing, Wastewater Industrial

Various

Caustic soda, Chlorine, Hydrochloric acid, Hydrogen

CHEM-GARD Horizontal Centrifugal Pump, FLEX-I-LINER

Sealless Self-Priming Peristaltic Pumps, Nonmetallic Tank

Pump Systems, SUMP-GARD Thermoplastic Vertical Pump

 

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Reprinted from World Pumps

 

Guide to the selection of materials for pumping

NaOH and NaOCl

 

This third article in an on-going series on the selection of materials for

pumping corrosive, abrasive and hazardous chemicals considers the

challenges presented by the caustic alkali sodium hydroxide and its

close relative sodium hypochlorite. George Black looks at the problems

and some practical solutions.

 

Sodium hydroxide and sodium hypochlorite

 

Sodium hydroxide (NaOH) is a corrosive alkali, commonly called caustic

soda or soda ash. It must be handled with care because it destroys

organic tissue and requires protection of the skin and eyes. It is derived

by electrolysis of sodium chloride, or by treating a solution of soda ash

with a solution of lime. It is widely used in the manufacture of other

chemicals, in the manufacture of detergents, pulp and paper, soap and

textiles, for regeneration of spent process solutions and for

neutralization of acidic wastewater.

 

Its sister chemical, sodium hypochlorite (NaOCl), is a salt generally

derived from electrolysis of a cold dilute solution of seawater. It is

unstable in air unless mixed with sodium hydroxide. NaOCl is usually

stored in what is known as Labarraque's solution, and is readily

recognized by its disagreeable, sweetish odour and pale green colour.

It is widely used for the bleaching of paper, pulp and textiles, for water

purification, for medicines and in conjunction with sodium hydroxide for

treating and neutralizing wastewater. Although both of these chemicals

area readily handled by polypropylene, temperature variations and

wear-related service conditions often suggest the use of vinyls and

fluoropolymers for pump construction.

 

Scrubbing systems minimize obnoxious odours

 

The process design engineers at CH2M Hill were charged with the

responsibility of providing this Miami, FL, facility with a multi-stage air

scrubbing system that would achieve 99.9% H•S and odour removal. In

addition, it would have to assure H•S discharge less than 0.1 ppm. In

conjunction with this mandate, the new system was to be designed to cut

maintenance and conform to the latest projected environmental regulations

for plant emissions.

 

Pump selection and material specification were critical to dependable

operation of the scrubbing towers and related equipment because in all stages of the system, both chlorine and caustic additions were required. The

chlorine feed was provided by sodium hypochlorite and the system

incorporated the flexibility to adjust and control the 50% truck concentration

down to 25% as needed.

 

The six recirculation scrubber pumps (Figure 1) were specified as ANSI

horizontal centrifugals suitable for delivering 650 gpm against a 38' TDH

over a temperature range from 55-110°F. To ensure that no metal

would be in contact with the corrosive NaOH, NaOCl and H•S fluids required

to handle the broad range of pH values from 3 to 12, all fluid contact

pump components were to be furnished in homogeneous, chemically

inert polypropylene or one of the fluoropolymers. The stainless steel

shafts were to be isolated from the fluid by a thick sleeve of Kynar®, the

PVDF fluoropolymer, and the mechanical seal was to be reverse

mounted so that the metal component would be out of the fluid area.

These pumps are driven by 25 HP, 1800 rpm, TEFC motors.

 

An additional ANSI PP horizontal centrifugal pump with the PVDF shaft

sleeve was specified to recycle the NaOH caustic. This 4x3x10 pump

was required to deliver 500 gpm against a 16' TDH. It is driven by a 15

HP, 1750 rpm, TEFC motor. The same requirement for no metal in fluid

contact set for the other centrifugals applies to this pump.

 

The pumps required to transfer the 50% NaOH caustic soda and 12.5%

NaOCl chlorine solutions from their 1500 gallon fibreglass (FRP) storage

tanks are vertical centrifugal units designed to deliver 200 gpm against a

13' TDH at a temperature range from 50-90°F (Figure 2). These pumps

are driven by 5 HP, 1800 rpm, TEFC motors. The basic pumps have

all-wetted components in the pump head furnished in PP and the shaft

sleeve in PVDF, but the vertical pump columns are also specified in

PVDF. The wetted bearings are nonmetallic, consisting of Vanite and

ceramic.

 

Since installation of the new scrubber system, the facility has been achieving

99.99% reduction in sulphide levels and 99.5% reduction in odour measured

by inlet and outlet H•S and odour values (ED50).

 

Packed tower scrubber neutralizes sulphuric acid

 

Field reports on the use of sophisticated Gaylord Foundry scrubbers in

conjunction with resin bonded core mould operations indicated

unusually high maintenance problems traced to pump failure under the

severe corrosive and abrasive nature of the caustic and acidic fluids

they had to handle. Not only did the wetted parts of the pumps have to

be inert to varying concentrations of the sulphuric acid and the sodium

hydroxide needed for neutralization, but they had to withstand impact

and abrasion from sand particles as well.

 

In a typical operation, the SO• gas coming from the core machine flows

upwards through a deep bed of polypropylene rings continuously wetted by a

5% by weight NaOH solution supplied from a fibreglass tank. In the original

design, an externally-mounted centrifugal plastic pump was used for the

continuous circulation. Repeated failure of the seals, and the messy cleanup

required, led to a design change. When an in-tank polypropylene sump pump

with an integral pump/motor shaft was designed into the system, the seal and cleanup problems were solved.

 

Here's how the revised Gaylord Foundry Equipment scrubbing system

works. Two very different nonmetallic pumps are used to assure

dependable delivery of the neutralizing caustic. Controlled feeding of

50% NaOH is provided by a flexible liner peristaltic type rotary pump,

instrumented to respond on demand from the pH probe in the tank.

The two components of this sealless pump in contact with the fluid are

the Teflon® pump body and the Hypalon® flexible liner. Continuous

circulation of the caustic solution in the tank is maintained by the

polypropylene sump pump, which has no seals or sleeve bearings in

contact with the fluid (Figure 3).

 

Automatic regeneration of the neutralizing solution so that the pH stays

at 8.5 is maintained in this manner. A portion of the spent solution is

discharged, energizing the caustic transfer pump to meter a controlled

quantity of NaOH. Make-up water is then added to maintain the sump

at its preset level. A similar Gaylord scrubber design for the foundry

industry uses a sulphuric acid-based scrubbing solution. Since the

pump materials are inert to both the acid and caustic, the same pumps

provide satisfactory service.

 

Maintenance problems solved with CPVC/PVDF pump

 

The Burbank Drum & Barrel Company of Glena Park, TX, reconditions

half-a-million 55-gallon steel drums per year. The process involves

immersing the drums in a stripping solution of sodium hydroxide,

steam flushing them with the same solution, and power rinsing them

with 20% sulphuric acid. In addition to the required resistance to the

chemicals, the pumps at the reconditioning station faced a number of

wear factors that resulted in costly downtime for repairs.

 

Service factors included: chemical heat generated by the addition of

acid and water makeup, frequent start/stop operation, and abrasion

from oxide and scale. Here's how the problems were solved. The

existing pump was replaced with a vertical centrifugal design that had

the high wear components, the casing and impeller, made of

precision-moulded polyvinylidene fluoride (PVDF, Kynar). The column

and its welded vertical support gussets were provided in chlorinated

polyvinyl chloride (CPVC).

 

This combination of cantilever design and high tensile strength

engineered thermoplastics with superior abrasion resistance where

needed did the trick. Pump downtime for repair has been completely

eliminated.

 

Advantages of converting from hydrated lime to liquid caustic

 

Ever since the Harpeth Valley Water Treatment facility in West Nashville,

TN, switched their manual granular hydrated lime neutralizing system to

an automated system based on the use of 25% sodium hydroxide,

production has been up and costs have gone down. A critical factor in

the success of the new system is the selection of magnetically driven,

sealless ANSI centrifugal pumps with no metal in contact with the

corrosive fluid (Figure 4).

Figure 3. This compact packed tower

scrubbing system relies on two very

different thermoplastic pumps: a

flexible liner peristaltic type rotary

pump to meter 50% NaOH on demand to the

vertical in-tank circulating pump, which

provides a continuous flow of 5% NaOH

scrubbing fluid to the tower.

Figure 4. Close-up of one of the two

sealless thermoplastic

magnetically-driven centrifugal pumps

automatically delivering the required

volume of 25% NaOH on demand.

Figure 5. Polypropylene plastic tank and

pump station with pneumatically-operated

plastic valves, used in processing

chlorine chemicals.

Copyright 2016 - Vanton Pumps (Europe) Ltd - All rights reserved

About Us

In the 1950, Vanton developed a revolutionary all-plastic pump for use in conjunction with the first heart-lung device. The design limited fluid contact to only two non-metallic parts: a plastic body block and a flexible liner. This was the birth of our Flex-I-Liner rotary pump. Its self-priming sealless design made it an industry standard for the handling of corrosive, abrasive and viscous fluids as well as those that must be transferred without contaminating the product. Vanton now offers the most comprehensive line of thermoplastic pumps in the industry.

 

 

Stay in touch

mail@vantonpump.com

(+44) 01260 277040

Vanton Pumps (Europe) Ltd.

Unit 4, Royle Park

Royle Street

Congleton CW12 1JJ

UNITED KINGDOM

www.vantonpump.com